CHIN's

Desperate circumstances call for desperate measures, as Jake, Holt, Amy, Terry, Rosa and Charles must ally themselves with old enemies.

Jake and Charles investigate a dangerous serial killer. Holt continues to spar with his rival, Commissioner Kelly. Terry finds out about whether or not he has passed the lieutenant's exam.

In order to distract Terry from the stress of his upcoming lieutenant's exam, Jake, Holt and the squad decide to hold the annual Halloween Heist on a new date, Cinco de Mayo.

When Gina returns to the Nine-Nine, Jake and Terry get drawn into her life. Holt learns that Charles’s son, Nikolaj, may be a genius. Rosa copes with an injury.

Jake and the squad must track down a hacker who has infiltrated the Nine-Nine's servers; Rosa deals with relationship issues; Hitchcock and Scully attempt to cook the perfect lasagna.