In Dreamcatchers, after Tracy killed her father and nearly killed the prison transport van driver and their prisoner, Donovan Donati, Theo began tracking Tracy's movements, keeping it a secret from the McCall Pack, who were doing the same thing. It would not be revealed until later that Theo was aware of Tracy's existence as a result of his partnership with the Dread Doctors, who made both Theo and Tracy into Chimeras. After overhearing Scott McCall, Stiles Stilinski, Noah Stilinski, and Lydia Martin discussing the fact that Donovan identified the killer as being Tracy Stewart, Theo left the crime scene to start his own investigation.
The next day, Theo was in Scott, Lydia, and Kira's AP Biology class when he once again overheard Liam Dunbar warning Scott that Tracy had showed up in his own sophomore-level history class, in the midst of a night terror and actively hallucinating. After Tracy warned Scott and Liam that "they're coming for all of us," she vomited a large amount of modified mercury and passed out, leading Scott, Stiles and Malia to take her to the animal clinic to be evaluated by the Druid Emissary Alan Deaton, which Theo presumably overheard as well.
Theo did not show up again until after Tracy had awakened and revealed that she was not the newly-bitten Beta Werewolf they initially believed, but a part-Werewolf, part-Kanima Chimera who had used her Kanima venom to paralyze Scott, Stiles, Malia, and Deaton. Theo helped the remaining McCall Pack members recover from their paralysis before begging the three to allow him to assist them in their hunt for Tracy, assuring them that by doing so, they would not be inviting them into their pack. Scott agreed to Theo's proposal, and the four headed toward the Sheriff's station, where Malia had gotten a head start after figuring out that Tracy would be targeting Natalie Martin, her guidance counselor. Unfortunately for them, by the time they arrived, Tracy had nearly killed Lydia with her sharp tail and had been subsequently killed by the Dread Doctors, who gave her an overdose of modified mercury. Knowing Tracy was past saving, Theo instead focused his efforts on saving Lydia's life with a tourniquet made out of his belt.
In Status Asthmaticus, after Theo forcefully performed the memory manipulation ritual on Lydia to find the location of the Nemeton, Theo used a large syringe-gun full of the Dread Doctors' green serum to resurrect four of the deceased Chimeras laid at the Nemeton by the Hellhound Jordan Parrish. Among the four resurrected was Tracy, who had been dead for several weeks, as well as Hayden Romero, Corey Bryant, and Josh Diaz. Seeing that they were all disoriented from their revival, Theo informed them that they were now part of his new pack, and that he was now their Alpha. When Theo turned to walk away, Tracy was one of the first to follow after him.
In The Last Chimera, Theo and Tracy were seen infiltrating Eichen House with Josh and Corey in flashforwards to several weeks in the future. Upon their arrival in the supernatural ward, Tracy immediately put Dr. Gabriel Valack in a choke-hold against the wall so that Theo could examine Lydia, on whom Valack had performed trephination a short time earlier, without intervention.
In Damnatio Memoriae, Theo brought Tracy to the hospital, where she prepared to kill the driver of the prison transport van who remembered her from the night she killed her father. Theo watched as Tracy injected her Kanima Venom into his IV, causing the man's diaphragm to become paralyzed and prevent him from breathing to the point of suffocating to death. Theo was visibly proud of Tracy's killer instincts but later was disappointed when she, Josh, and Corey were unable to defeat Scott McCall in a fight.
In Codominance, Tracy and Theo tracked the Beast down to the school while the two discussed the Beast and the yet-unknown Chimera teenager underneath. Tracy then asks Theo about the original La Bête du Gévaudan, leading the two to discuss how the Dread Doctors brought it back and how many people he killed during his first run. While they're talking, they heard the Dread Doctors approaching them. Tracy managed to attack one of the Doctors but was knocked out almost immediately afterward. Theo then explained to them that he was not happy with his pack and was clearly insecure about the fact that he was neither an Alpha nor a real Werewolf. However, Theo became scared when the Beast appeared behind the Dread Doctors with the severed head of its most recent victim in its hand. Just as Tracy began to regain consciousness, the Beast stepped toward them and roared so loudly that Theo and Tracy had to cover their ears. When they finally were able to uncover them, they found that the Beast and the Doctors had vanished.
In The Sword and the Spirit, after his encounter with the Desert Wolf, Theo returned to the operating theater to find his pack, including Tracy, all nursing severe injuries, though they managed to incapacitate Deucalion long enough to put a wolfsbane IV drip in his arm. Theo was clearly disappointed by the fact that three Chimeras were unable to defeat one blind Alpha Werewolf, but he became distracted when Tracy revealed that Deucalion was aware of their search for the Beast.
In Amplification, the flashfowards from the midseason finale are finally caught up with the current time. Tracy and Theo, along with Corey and Josh, managed to break into Eichen House and get to Lydia's room but before they could escape with her, Parrish showed up in Hellhound form, growling and melting through the door to the supernatural ward with his pyrokinesis.
In Lie Ability, Theo and Tracy watched as Parrish, in full flaming Hellhound-mode, entered the supernatural ward. Theo tried to get his so-called pack to attack him, but only Tracy did so without needing multiple repeated orders. When Theo saw Tracy get knocked to the ground after her venom failed to paralyze Parrish, he insisted that Josh and Corey follow suit; Josh's electrokinetic powers also had no effect on Parrish, and though Corey managed to stop one of his punches with his super strength, he ended up being burned over most of his body in a huge blast of fire. Seeing Tracy knocked out on the floor, Theo decided to leave her and the others in favor of helping Stiles find Lydia. Later that night, Tracy, intent to impress Theo, tried to capture Lydia for him after the McCall Pack successfully broke her out of Eichen House, but she was incapacitated by Natalie Martin, who used one of the tasers belonging to the guard Tracy killed to electrocute her.
In The Beast of Beacon Hills, Theo and Tracy were in the lair with Josh and their captive, Deucalion, where both Tracy and Theo tried to convince Josh to put on the Dread Doctor's Mask. When Josh refused to do so out of fear of what it could do to him and tried to leave, Tracy watched as Theo took Josh's pain and power before killing him. After Deucalion questioned Theo's decision to kill one of his few remaining "Betas," Tracy remarked that Josh was small and inexperienced before reminding him that, now that he had Josh's powers, he could wear the mask himself. Tracy later made an alliance with the Desert Wolf to get her into the mountain ash barrier protecting the McCall House so she could kill Malia Tate, the Desert Wolf's Werecoyote daughter who Theo has shown a great amount of interest; this alliance may or may not have been on Theo's orders.
In Apotheosis, after being wounded by the Beast, Tracy brought Theo back to the operating theater. While Tracy comforted Theo, the two eventually kissed, but Theo was ultimately in too much pain to continue for more than a few moments. Tracy lamented the fact that she did not possess the power to take the pain of others before suggesting that Theo take a few days to rest, recover, and come up with a new plan. Theo was appalled by this suggestion and tricked her into thinking that he was romantically interested in her before taking advantage of her distraction and stealing her power; unlike Josh, Tracy fought back, but was futile, and Theo ended up killing her. Though Theo used her power to successfully paralyze Scott and Liam, the venom ended up not having any effect on the Beast, preventing Theo from killing and stealing its power.