A group of robbers kill a security guard who used the APB app before his death; Gideon redesigns a feedback chair, for use in the investigation room; Murphy learns that her ex-husband may be watching them.
Gideon and Martinez want to employ a new technology to help their case after a man's extreme actions for justice are called into question; the assistant State Attorney continues his interrogation of getting about the 13th District.
Gideon opens a nerdcon as the station goes back to basics. While at the nerdcon a threat is revealed on Gideon's life by a hacker who impressed Gideon. Brandt and Coss track a suicidal student , while secrets are revealed.
After another fire hits one of the gang-run buildings, Murphy and Gideon find out that the cause is much worse than an act of gang retaliation. Meanwhile, Goss is unsure about Brandt's old service buddy, after he's accused of assaulting a customer at the bar where he's a bouncer. Goss seeks the truth and Gideon is faced with an important decision.
An FBI agent seeks Murphy's help when he learns his informant's life may be in danger. Gideon not only assists Murphy with the investigation, but makes a decision that could make or break his company. Meanwhile, the past comes back to haunt Ada.
When a serial rapist is on the loose, Captain Conrad orders Gideon and Murphy to find the perpetrator before there are any more victims. However, after Gideon's new technology fails to prove helpful for the case, the team is forced to turn to an unexpected ally for help. Meanwhile, Ada struggles with a threatening part of her past and worries that if it resurfaces, it may be harmful.
Gideon's life is threatened, so Conrad sends him home to get away from District 13. When the men after Gideon find him, Murphy and Gideon hide in his secret lab, where they become trapped. As they search for a way to escape, Murphy learns the main reason these men are after Gideon.