|“|| "I promise everything will get back to normal, Lydia. All that you have to do is everything that I ask. [...] Timing is key here, Lydia. It all needs to happen by the next full moon. Do you know what they call the full moon in March? It's called the Worm Moon. They call it that because that's the full moon of winter, and the worms would literally crawl out of the earth as it thaws. Kind of has the feel of a rebirth, doesn't it?"|
"But the full moon is on Wednesday. That's my birthday."
"Exactly. And Lydia's birthday is always the party of the year, isn't it? Everyone wants to go to this party. So, we're going to make it a very special party..."
Stiles realizes that all the murder victims were on the Beacon Hills High swim team and the coach at the time was Isaac's father. At Lydia's birthday party, Lydia spikes the punchbowl with wolfsbane petals, causing all the party attendees to hallucinate the things they fear most. She attacks Derek and brings him to the Hale house, where Peter uses Derek's blood to come back to life. Victoria kills herself so that she would not become a werewolf. It is revealed that Matt is the Kanima's master.
- Tyler Posey as Scott McCall
- Crystal Reed as Allison Argent
- Dylan O'Brien as Stiles Stilinski
- Tyler Hoechlin as Derek Hale
- Holland Roden as Lydia Martin
- Colton Haynes as Jackson Whittemore
- JR Bourne as Chris Argent
- Linden Ashby as Sheriff Stilinski
- Ian Bohen as Peter Hale
- Michael Hogan as Gerard Argent
- Seth Gilliam as Dr. Alan Deaton
- Daniel Sharman as Isaac Lahey
- Stephen Lunsford as Matt Daehler
- Eaddy Mays as Victoria Argent
- Gage Golightly as Erica Reyes
- Sinqua Walls as Vernon Boyd
- Brian Trapp as Drag Queen (as Brian "Phoenix" Trapp)
- Mike Crowley as Kid
- Shantal Nyree Rhodes as Danielle
- Argent helps Victoria commit suicide after she was accidentally bitten by Derek during a fight and turned into a Werewolf in Raving. It was revealed that a Hunter who is bitten must take their own life before their first transformation in Ice Pick.
- It is revealed that Lydia's birthday is in March when she throws herself a birthday party under the orders of Peter Hale, whose spirit was bound to her after he bit her in Formality and subsequently died in Code Breaker.
- Peter is resurrected in this episode by Lydia, using a ritual that involves the full Worm Moon (the name for the full moon in March) and a blood sacrifice from Derek. He was first killed by Derek in Code Breaker.
- Claudia Stilinski was referenced by Sheriff Stilinski in Stiles' hallucination. She was last mentioned in Wolf's Bane.
- Matt Daehler is revealed to be the Kanima master when he was thrown into the swimming pool at the party and was unable to swim. It was first learned that the Kanima master was afraid of water in Raving.
- This episode marks the first appearance of Danielle, who won't appear again until Chaos Rising.
- It is revealed that purple wolfsbane, when ingested orally by humans, Werewolves, and Kanimas alike, will cause horrifying hallucinations, as shown with Scott McCall, Stiles Stilinski, Allison Argent, and Jackson Whittemore.
- It is revealed that the Hale Family's symbol, the Triskelion, is meant to represent the ranks of Alpha, Beta, and Omega in the Werewolf hierarchy, reminding them that they can always rise to one or fall to another. This symbolism will play a role in the story throughout the rest of the series.
- Derek refers to the concept that helps a Werewolf remain human during full moons and times of intense stress/emotions as an "anchor," which was first alluded to in the Season 1 episode Heart Monitor, when Scott used Allison as his connection to his humanity. This is also the first episode in which Derek admits that his anchor is anger (which will be referenced in future episodes), and in which Isaac admits that his anchor is his father before he became abusive toward him.
- The drag queens that Scott and Stiles met at Jungle in Frenemy appeared in this episode after Stiles invited them to the Martin House when it became clear that no one wanted to attend Lydia's party.
- The title is a play on words, replacing the "guest" in "party guest," which in reference to Lydia Martin's birthday party, with "guessed" as a reference to the ongoing speculation as to who is the Kanima master, which was finally revealed to be Matt Daehler.
- Martin House
- Lydia's Bedroom
- Back Patio
- Beacon Hills High School
- Lacrosse Field (dream only)
- Locker Room
- Argent House
- Allison's Bedroom
- Railway Depot
- Daehler House
- Stilinski House
- Stiles' Bedroom
- Hale House
- Beacon Hills Memorial Hospital