The episode picks up right where Lies of Omission left off, with Scott, Liam, and Theo at the Beacon Hills Animal Clinic with Hayden Romero, who is seemingly dying from an overdose of the Dread Doctors' Modified Mercury. Scott has just rejected Liam's request to give Hayden the Bite, much to Liam's anger. When Liam begins to argue that Scott promised to do everything that he could to save her (a promise made in Season 5's Strange Frequencies), Scott insists that giving her the Bite would likely kill her, since she's already weakened from the mercury (and Peter Hale implied in Season 3's De-Void that those who have been weakened by illness or injury are unlikely to survive the Bite) and they have no idea how it will affect the transformation process.
Liam's argument sends Scott into an anxiety-induced asthma attack, causing Theo to toss him a mysteriously-procured inhaler, which Scott immediately uses to subdue his wheezing, too overwhelmed by the fighting to question it. Scott states that he has another idea of how to save Hayden, and Theo, who is holding the weakened upright, insists that they need to figure it out soon due to her rapidly-deteriorating condition.
The scene cuts to Stiles, who is frantically driving through Beacon Hills in his Jeep. He becomes overwhelmed with emotion when he looks over and sees the bloody wrench he used to fight Donovan Donati (in Season 5's A Novel Approach) sitting on the passenger seat. Suddenly, the Jeep’s engine overheats and fills the inside with smoke, forcing Stiles to pull over in a residential area to check it out. Stiles goes to fix it with his new toolbox, but when he sees the empty space where the blood-covered wrench should be, he becomes so upset that he grabs the wrench and chucks it at the windshield, shattering it before he collapses onto the pavement in despair.
Meanwhile, at the police station, Sheriff Stilinski, Valerie Clark, and several other deputies are communicating with dispatch over the radio about reports of a large animal or man running around town. Deputy Clark informs them that she sees the suspect and describes it as "loping" on all fours like an animal as she chases behind it. She pursues it so closely that she ends up totaling her cruiser.
Knowing that the suspect was heading for the high school, Clark runs nearly a full mile to get there, where she finds the entrance hall destroyed after something threw the stone school sign through the roof, creating a large hole in the ceiling and crushing the electrical wiring. She meets up with the Sheriff, who assures her that her life is more important than her police cruiser before the two assess the damage to the school. The two don't notice the nearly-full supermoon through the roughly-made skylight above them, or the Dread Doctors standing around the corner behind them, who have just determined that the last Chimera's success is imminent, though who or what the Chimera is remains a mystery.
The next day, at the animal clinic, Melissa has just arrived with a bag of chelation solution, a treatment used intravenously to to remove heavy metals such as mercury from the blood, in hopes that they can use it to save Hayden. However, the supermoon's affects is already affecting Liam, who is already barely keeping control over himself as a result of his fear of losing Hayden; when Hayden winces as Melissa inserts the IV line into her hand, Liam roughly grabs Melissa by the wrist, hurting her until Theo advises him to let go. Theo reminds them that they're trying to save a life, not kill each other, before they discuss the upcoming supermoon that night.
Out in the clinic's reception area, Theo and a despondent Scott sit in the chairs and discuss their current dilemma. Theo brings up the fact that they're going to need to keep an eye out for Liam, reminding him that Hayden is Liam's first love before asking Scott if he remembers what that is like; Scott, heartbroken over the reminder of his first love Allison Argent's death and his current love Kira Yukimura's absence, insists that he remembers. Theo continues on to say that Scott needs his pack to fight the Dread Doctors and the last Chimera, and Scott, devastated by everything that has happened recently, states that he's not sure he has one anymore, leading Theo to assure him that he will talk to the rest of the pack members on his behalf.
Back at the Sheriff's station, Stilinski finds an envelope on his desk and opens it to find Stiles' school library card with a note from Melissa McCall stating that the maintenance staff found it in a trashcan at the hospital. (For reference, Stiles, afraid that his father would find out that he was in the school library when the so-called "prank call" occurred-- which in reality was Stiles calling the police after he accidentally killed Donovan-- threw the card away in Season 5's Ouroboros). The appearance of this card causes Stilinski to become confused about what this means for Stiles and his involvement the night of Donovan's death.
At the clinic, Hayden, fearing the worst, asks the pack members to find her sister, Valerie Clark, stating that she didn't want to die without her sister there. When they are unable to reach Valerie by phone, Scott assures her that he will find her himself and leaves to do so, heading first for the high school due to the news that the deputies were still investigating the damage to the entrance hall by the last Chimera.
At the station's holding cells, Lydia is sitting on the floor outside of the cell where Parrish locked himself up as a result of the revelation that he had been going into trances as a result of his still-unknown supernatural nature and stealing the bodies of the dead Chimeras in order to bring them to the Nemeton. The two begin discussing their connection, with Parrish remarking that he, like Lydia, is drawn to death and wondering aloud if he's some kind of Banshee. When Parrish asks Lydia if her powers ever scare her, but Lydia replies that they scare her less now that she knows more about who and what she is.
He then informs her that he had a vision of Lydia appearing after he was attacked by Belasko in Creatures of the Night, and that he was initially afraid that she was there because he was about to die. Lydia laughs and points out that she was at the high school that night, trying to stay out of the rain, before she references the Wild Hunt and their connection to storms like the one that was occurring the night Parrish was talking about, which Kira told her about earlier on. This causes Lydia to have a revelation when she describes the Ghost Riders's horses and hounds, and she quickly informs him that she needs to research something and will be back soon.
Over in the residential area of Beacon Hills, Malia has just arrived to pick up Stiles where the Jeep broke down the previous evening. She looks concerned as Stiles sadly watches a tow truck pick up the Jeep to take it to the shop before asking him why he let the Jeep's condition get this bad.
Stiles points out that he's been a little preoccupied with everything going on lately,
to which Malia replies that she has noticed, crytically adding that she notices a lot more than he thinks. When the two get into Malia's car, Malia asks him if she's taking him home, but Stiles instead asks to be taken to the Sheriff's station because he has something he needs to tell to his dad.
Once they get there, Malia, in a shocking twist, asks Stiles if he plans on telling Sheriff about Donovan, which shocks Stiles, who had no idea that anyone aside from Theo knew what had happened. Malia admits that she saw the bite on his shoulder when he was sleeping and put it together before adding that she never said anything because it doesn't matter to her. However, Stiles insists that it matters to him, getting out of the car and heading into the station without another word and leaving a sad and confused Malia behind.
When Stiles enters the Sheriff's station, he is told that Stilinski had left and would return in a half-hour, leading Stiles to decide to wait for him in his office. Meanwhile, Parrish is laying on the bench in his holding cell, sleeping soundly on his back and dreaming about bringing more dead Chimeras to the Nemeton. At the front desk, the deputies on duty begin sweating and remarking how hot it was in the building, not realizing that it was Parrish's power of Thermokinesis that was increasing the temperature. Suddenly, Parrish's eyes fly open, revealing his fiery, glowing reddish-orange irises.
At the high school library, Lydia has just found a book on mythology, where she reads an excerpt aloud to herself: "Woden's Hunt, also known as the Wild Ride or [[Wild Hunt. A myth of devilish riders in the sky accompanied by black dogs, spectral beasts whose eyes glowed with fire. A Bearer of Death and Guardian of the Supernatural, the black dog is also know by its more common name-- the Hellhound." Just as Lydia realized that Parrish is indeed a Hellhound, she experienced an auditory premonition of Scott and Liam fighting each other in the library, with Scott anxiously pleading, "I can't let you kill me!"
Before Lydia can even react, Theo appears behind her and states, "You know what's coming, don't you?" Lydia informs him that someone is going to die there tonight, which Theo confirms, though he adds that he can't let her tell anyone. Lydia, confused, asks him why, Theo smiles and tells her that he wants her and the others in the McCall Pack before snarking that he'll give her time to think about it and backhanding her across the face so hard that he knocks her out.
Back at the Sheriff's station, Parrish, in Hellhound-mode, breaks himself out of his cell by using his thermokinetic abilities to melt the bars and pull them apart before leaving to go retrieve the newest bodies (Corey Bryant and the unnamed female Chimera killed by Kira Yukimura in a fugue state) and take them to the Nemeton.
Melissa, with help from Mason Hewitt, has brought Hayden to the hospital, bringing her down to the morgue in hopes of discretely giving her further treatment when the chelation has no effect. Mason and Melissa become concerned when they see jaguar spots that look like bruises appearing on her neck and chest and grimly admit that she's getting worse.
Some time later, Malia, who has come to her old coyote den in the woods to think, looks at her sister Kylie's baby doll when sudden, a dark brown wolf trots into the den behind her. Malia looks at Theo in shock and asks him how he's able to full-shift, and he tells her he can show her how. Malia remarks on Theo's shameless nakedness, but he retorts that he has nothing to hide before asking her if Scott knows she's planning to kill her mother, the Desert Wolf. When Malia shoots him a look, he assures her that he's not judging her, and that he actually wants to help her, and that if Scott isn't willing to do so, then someone else needs to take the lead. He goes on to inform her that Melissa and Mason were taking Hayden to the hospital and that there were still two more dangerous Chimeras that needed to be taken care of before insisting that the pack needs her help.
However, when Theo asks Malia to come with him, he sneakily locks her up in a room in the wing that is currently under construction at the hospital, which serves to prevent her from interfering with his plans for Scott, just like how he incapacitated Lydia, knowing that there was no way he could convince her to turn on Scott and join him. Theo then throws away the key to the supply closet in a sewer at the high school, where he then makes his way to line the majority of the library with Mountain Ash, both inside the library and outside of the building.
At the Sheriff's station, Parrish, still in Hellhound-mode and in a trance, walks into the bullpen, causing the deputies to aim their guns at him and demand that he stop what he's doing. However, Stiles, not wanting anyone to get hurt, jumps in and begs them to stand down, knowing that the Hellhound, while not an active threat, will fight back against those who get in the way of his mission. When Parrish peacefully walks out of the station without a word, Stiles follows after him to see what he's planning to do.
Scott, who was unable to find Valerie for Hayden, received a text message from Lydia's phone that told him to go to the library. Once there, Scott finds Lydia's phone laying on a bookshelf, which, when combined by the fact that he caught the sight of an unfinished Mountain Ash barrier, caused him to realize he had just walked into a trap. Unfortunately, he was unable to escape before Theo appeared and finished the barrier, effectively trapping Scott inside the library. Scott, dumbfounded, said that Theo handling Mountain Ash wasn't possible just before realizing that Theo is actually a Chimera, a fact that Theo confirmed before smirking and informing him that he's the first Chimera and is both part-Werewolf and part-Werecoyote.
He commends Scott on his use of the word "Chimera" to describe them, because not only does it mean "a creature with incongruous parts," (an apt description of humans turned into pseudo-supernatural hybrids), but it also means an "unrealizable dream." Scott assumes that Theo must be the successful Chimera due to the fact that he is both alive and unaffected by the rejection process that all the other Chimeras went through, but, Theo, much to his chagrin, admits that while he successfully transformed into a Chimera, he wasn't what the Dread Doctors ultimately wanted, adding "We can't all be True Alphas." When Scott lunges for Theo and is thrown backward by the force of the barrier, Theo laughs and says that even he felt the pain from it before turning on Parrish's cell frequency jammers so that Scott can't call anyone for help and bidding him good luck in "what comes next."
Overwhelmed and uneasy about Theo's threat, Scott sees the door to the stairway to the roof is open and heads to it in hopes of finding a way to escape. However, he finds that the mountain ash barrier exists outside of the library as well as inside, preventing him from jumping down to safety. He attempts to try to break through it (like he did in Season 3's Lunar Ellipse) but the exertion quickly triggers yet another asthma attack and forces him to use the inhaler in his pocket, and Scott is too preoccupied to remember he got it from Theo. After his wheezing has subsided, Scott becomes so angry and frustrated by what has happened in the last few days, he screams as he crushes his inhaler into a twisted piece of metal in his hand, releasing a cloud of purple vapor to be released. This causes Scott to realize that Theo has been weakening him for weeks by giving him purple wolfsbane-laced inhalers, which is why his asthma returned despite his Accelerated Healing powers typically rendering that impossible.
Scott looks up at the supermoon overhead for a moment until a growling noise catches his attention. It's Liam, who is shifted into his Werewolf form and is glaring at Scott with loathing, indicating that the supermoon in conjunction with the stress of Hayden's illness is heightening his anger and aggression to overwhelming levels, and that Theo allowed Liam entry before resealing the library with Mountain Ash. Liam reveals that he intends to kill Scott so that he can become an Alpha and give Hayden the Bite himself in order to save her before tackling him, causing the two to fall through the library skylight and land on one of the library tables at the ground level.
Meanwhile, at the hospital morgue, the chelation therapy continues to do nothing to help Hayden recover from the mercury poisoning, to the point where Mason and Melissa begin to believe that this has made her worse. Realizing that her death is imminent and that Scott has yet to return with Valerie as Hayden requested, Melissa sends Mason to go get Liam so he can say goodbye to his girlfriend, not knowing that he's in the middle of a smack-down, drag-out fight with her son and his Alpha.
Back at the library, Scott is struggling with the battle going on between himself and his only true Beta Liam, as he's both weakened by the weeks of unknowingly inhaling purple wolfsbane and is desperate not to seriously hurt Liam, as he knows the pain of potentially losing one's first love and understands that the full moon is making him behave this way. When Scott is unable to incapacitate Liam, he slams Liam to the ground before running to the wall of windows, bathing himself in the light of the supermoon and finally allowing himself to shift into his Werewolf form, an urge he has been repressing all day.
Scott goes on to tell Liam that he can't let him kill him, just as Lydia heard in her premonition earlier in the day, pointing out that Theo needs Liam to kill him and steal his power, because a True Alpha's power can only be taken by a Beta of his own making, and that if Liam does kill him, Theo will then kill Liam to take the Alpha power for himself. Unfortunately, Liam is still too overwhelmed by grief, stress, and heartbreak amplified by the moon to listen to him, and he insists that Theo isn't forcing him to kill Scott-- he wants to do it himself. Their battle starts up again, even more brutal than before as Liam attempts to kill his own Alpha.
After hours of pounding on the door, Malia finally breaks herself out of the room Theo locked her in, only to find a trail of what appears to be blood on the floor. She uneasily follows it to find a brand new Chimera, Noah Patrick, feasting on a bag of donor red bone marrow in the wing that is currently under construction. She immediately shifts into Werecoyote form and starts to fight him; though she initially overpowers him, he demonstrates the power to extend bone claws out of his wrists and hands and uses them in the fight, swiping Malia across the cheek with one and gaining the upper-hand.
Fortunately for Malia, Braeden, having just arrived back in Beacon Hills, appears and briefly incapacitates Noah by shooting him with a taser bolt from her modified military-grade stun gun; though it does not harm Noah for long, it scares him enough to snap himself out of his violent state and flee the scene. Braeden then looks at Malia in exasperation before asking, "What the hell was that?" As the two make their way outside in the parking lot, Braeden reveals that not only does the Desert Wolf know that Malia is alive, but she knows Malia plans to kill her and is now coming to Beacon Hills.
Elsewhere, Stiles is continuing to tail Parrish, who has just arrived at the Sheriff's Department Impound Lot, where Stilinski had hidden the bodies of Corey Bryant and the unnamed female Chimera found at the McCall House. Just as Parrish found and taken the bodies and left to go to the Nemeton, Stiles calls Theo to ask for a ride. However, when Theo arrives, he brings Stiles' library card, which he found in Stilinski's office; Theo remarks that even the son of a Sheriff can make stupid mistakes and mentions that when Stilinski was looking for Stiles, he ran into Theo instead, implying that he had caused Stiles' father harm and revealing to Stiles that Theo was playing him and the rest of the McCall Pack the whole time.
Theo goes on to say that he was telling the truth about coming to Beacon Hills in order to find a pack, and insists that he wants Malia ("the Werecoyote whose first instinct is to kill"), Lydia ("the Banshee surrounded by death"), Kira Yukimura ("the Dark Kitsune"), Liam ("the Beta Werewolf with anger issues") and finally, Stiles, who he refers to as Void Stiles. Theo goes on to add that unfortunately for the others, the pack he wants doesn't include Scott, implying that his intention is to become a real Werewolf and an Alpha so he can lead the McCall Pack's powerful and resourceful members.
When Stiles becomes overwhelmed by anger, Theo remarks that his pulse is rising, but not because of fear, and adds that though the Nogitsune is no longer possessing him, Stiles still has more blood on his hands than anyone in the pack as a result of all those who died due to the Nogitsune's machinations in his body. Stiles furiously retorts that he's "about to get more" before lunging toward Theo and tackling him as he punches him in the face numerous times. This attack only causes Theo to laugh as he gets back to his feet, revealing that his intentions have always been for Stiles and the rest of the pack members to embrace their violent dark sides. He then informs Stiles that he has a choice to make-- he can either find and save his father, or he can find and save his best friend, Scott, though he cruelly reminds him that they no longer seem to be best friends and that Stiles never needed him anyway. When Stiles becomes enraged once again, Theo assures him that he can still save his father, but only if he leaves Scott for him.
As expected, Stiles chooses his human father, leaving Scott to be overpowered and nearly mauled to death by Liam, who was only stopped from giving the killing blow by the arrival of Mason, who managed to break through Liam's rage and stop him. He then sadly informed Liam that Hayden had died, causing Liam to panic so badly that he shifted back into human form and rushed to the hospital to see her one last time. Mason stays behind to pick up the exhausted and badly injured Scott, and when he asks Scott what happened, he simply states that it was the supermoon. Mason was about to remark that the events of the last day were "bad timing" when suddenly, Theo arrives, furious at the fact that Liam has left and Scott is still alive, and finishes the sentence for him. Theo then aggressively asks Mason why he couldn't have waited five more minutes before backhanding him hard enough to knock him out.
He turns to Scott, who, without Mason's support, is swaying on his feet, and states that he should have stayed to make sure Liam would finish the job. Scott, who at this point, is so depressed and self-loathing regarding his fractured relationships with his packmates that he's passively suicidal, smiles and replies, "Because now you're going to have to kill me yourself." When Scott adds that the pack is still is, Theo admits that they may not be his own right now, but is sure that they will eventually come around. Scott maintains that they'll never follow him, and when Theo, in a rare display of his own insecurities about his pseudo-supernatural nature, retorts, "Because I'm a Chimera? Because I'm not a real Werewolf?", Scott weakly smirks and states, "Because you're barely even human." This remark angers Theo so much that he flicks out his claws and plunges them in the space under his sternum and between his ribs, twisting his hand viciously before pulling his hand out and allowing Scott to fall to the floor. After a moment, the glowing crimson light fades from his eyes, indicating that Scott has died, ruining any chance of Theo being able to steal Scott's powers. Mason, finally awakening from his concussion, frantically crawls over to Scott's body as Theo leaves the library.
Back at the hospital, Liam has just made it to the morgue in the closed wing and is sitting on the floor, cradling Hayden's dead body in his arms as he cries. After a moment, Parrish, in full-Hellhound-mode with eyes blazing with reddish-orange fire, silently appears in front of him and waits patiently as Liam, heartbroken, leans down and kisses Hayden on the forehead before reluctantly allowing Parrish to take her to the Nemeton without protest.
Melissa has just arrived to the school to find Mason kneeling by Scott's body, dazed and devastated as he silently stares at his blood-soaked hands. Melissa immediately goes into denial mode as she assesses his injuries, murmuring "No, no, no, no, no..." When Melissa begins to perform CPR, Mason, who is clearly just as distraught as Melissa, sadly informs her that he already tried, and that he hasn't had a pulse in over fifteen minutes, but Melissa is not deterred by this information. He gently reminds her that bringing someone back who has been dead this long is impossible, Melissa argues that Scott isn't someone-- he's her son, and an Alpha, and she maintains that he's too strong to die like this. She continues to alternate between chest compressions and rescue breaths as she begs Scott to wake up and roar, and just as she and Mason are close to giving up, Scott, revived by his mother's loving efforts and the power of the supermoon, awakens and roars at the top of his lungs, causing both Mason and Melissa to stare at him in awe.
Meanwhile, Lydia wakes up in the Doctors’ operating theater, finally revealing what events occurred to cause her to be admitted to Eichen House in the flashforwards in Season 5's premiere Creatures of the Night— the untrained Theo uses the Werewolf memory sharing ritual on Lydia for the first time in order to learn the location of the Nemeton. While the ritual is successful in that Theo gets the information he was seeking, the trauma causes Lydia to slip into a catatonic state.
At the McCall House, the still-injured Scott is laying on his bed as Melissa tends to his wounds. He looks devastated as he admits that he lost this battle, but when Melissa reminds him that all leaders suffer loss that they believe is unbearable, Scott insists that this time, he lost everyone. She assures him that he'll get them back, and when Scott despondently wonders why they would ever come back to him, Melissa argues that even when a leader has lost everything they have to give, they still have one thing left-- hope. She urges him to give them hope as Scott ruminates on the events of the last few days.
Theo, with Lydia's barely-conscious body in one hand and a syringe gun filled with the Dread Doctors' green serum in the other, has finally made it to the Nemeton, where he exclaims, "You think you've lost your mind, Lydia?
Well, watch this!" and walks toward the pile of bodies on and around the Nemeton, dropping Lydia onto the ground in the process. He then takes the syringe gun and injects a significant amount of the serum into the necks of Josh Diaz, Corey Bryant, Tracy Stewart, and Hayden Romero, causing them all to be resurrected and awakened back to life. When the confused Chimeras ask what happened, Theo informs them that he is their Alpha, and they belong to him as his new packmates before walking away, leading the Chimeras back to town and leaving the still-catatonic Lydia behind to fend for herself.
In the finale’s final moments, the Dread Doctors return to the operating theater, where The Surgeon uses his sword cane to break through the tile wall, revealing a fresco painting of what appears to be a bestial Werewolf fighting against a flaming Hellhound. The Doctors then begin to chant "La Bête. La Bête. La Bête.", revealing that they may have resurrected the famed Beast of Gevaudan.