A group of robbers kill a security guard who was a former police officer and who, before his death, used the APB app. Meanwhile, Gideon redesigns a feedback chair - previously utilized in the aerospace industry - for use in the investigation room, and Murphy learns that her ex-husband, Assistant State Attorney Scott Murphy, may be watching them.

After armor-piercing bullets are found on the black market, the team decides to send Officer Brandt on an undercover mission to catch the criminals selling them. Meanwhile, a task force led by Murphy's ex-husband threatens Gideon's future.

After a man goes to extremes to get justice for his daughter, Gideon and Martinez use new technology to investigate the case. Meanwhile, the assistant state attorney continues to interrogate Gideon and his ideas for the 13th district in the all-new.

After Gideon's father finds himself in trouble, he turns to Gideon for assistance. Meanwhile, Brandt and Goss work to track down the mastermind behind a huge grand theft auto case.

When Murphy and Ada team up to help solve a murder, they discover that the killer's next target is Gideon. After learning his safety's at risk during his company's huge technology convention, they utilize a high-tech scanning system to try and track down the killer.

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.

When Gideon is blamed for recent terrorist attacks, he and Captain Conrad work fast to track down the real culprit. But Gideon finds himself in a predicament, when Danny continues to use his technological skills to disrupt the city, leaving Murphy, Ada and the rest of the squad struggling to find a way to stop him. Discover who makes it out alive.