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’ 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.
Holt and Jake investigate a case at Kevin’s university. Terry and Amy take the squad out for lunch.
Work is so busy for Jake and Amy that they end up celebrating their anniversary while standing guard over a comatose patient in the hospital.
Charles brings Jake into a case when a therapist reports one of his patient’s missing. Holt finds out Rosa has a new girlfriend that everyone else has met. Amy believes she has accidentally received a package meant for Terry.
A visitor from Latvia causes Charles and Jake to investigate Nikolaj's family. Holt and Amy bring in famed forensic scientist Dr. Yee to help close a case for Rosa.
Jake helps Amy navigate the complicated relationship she has with her brother, David. Charles introduces some drama into an undercover mission involving Terry and Holt.
Jake and Amy investigate a difficult "he said, she said" case. Holt becomes suspicious after learning his lifelong arch nemesis died in a prison transport accident.
Jake and Holt clash over the merits of a new hire in the 99th precinct. Terry, Amy, Rosa and Charles try to do something about the cluttered bullpen.
Jake and Rosa must deal with a mother in mourning as they work and rework a crime scene in order to solve a difficult murder case with confusing evidence.
Terry believes the Pontiac Bandit is stealing cars again, but Jake is convinced his friend wouldn't do that. The rest of the squad fight to keep Shaw's a police bar after some firemen stake their claim to it.
Gina spends time with each member of the squad.
Jake and Gina attend their 20th high school reunion. Charles helps Rosa with her love life.
Jake and Charles investigate a case of Hitchcock and Scully's from the 1980s. Amy's uniformed officers and Terry's detectives fight over limited resources.
Jake and Amy go on their honeymoon. Capt. Holt finds out whether he is the new commissioner of the NYPD.
