Additional clues about Shay's death emerge. Meanwhile, Severide faces an old foe; Shay's sister arrives, inspiring the firehouse members to reflect on her life; Otis' grandmother relocates; and Boden sets out to help his dying father.

The crossover continues on Chicago P.D. S02E13 A Little Devil Complex (II).

Herrmann tries to rescue a toddler from a structure fire. Mills discovers unexpected news.

The squad members come together to support one of their own. Herrmann tries to figure out how to reopen Molly's after it gets shut down. Severide digs deep into the arson investigation.

Severide confronts Chili about her erratic behavior. Members of the firehouse rally around Casey, who they think would make a great candidate for alderman. Jimmy squares off against Antonio in the first annual "Battle of the Badges" boxing match.

Lt. Casey finds himself in a bad spot when a forklift accidentally pierces a tank storing acid. Casey is forced to use extreme measures to help a father and daughter in peril. Dawson and Brett respond to an elderly woman who has a difficult senior advocate. Mouch and Brett continue with their secret project. Otis shares some exciting news as he prepares himself for an unusual expedition.

The team is on high alert when a previous incident during a fire call leaves a dangerous man on the loose with a vendetta to target Lt. Casey and Firehouse 51, and prompts CPD to step in. Dennis Mack, a buddy firefighter of Severide's father, comes to town to shadow the squad but may have ulterior motives. Meanwhile, Brett and Antonio's relationship hits some bumps in the road when his ex-wife Laura surfaces and Casey has a present for Dawson.

Casey goes to drastic measures to help his friend Kannell. Lt. Severide helps an old widow come to terms with letting go, but realizes he may be the one needing help. Otis and Cruz deal with surprises when they find a new roommate to move in. Dawson and Brett get crafty to save a young girl.

Lt. Casey gets off on the wrong foot with Sam Mullins, a temporary Chief who is filling in for Boden at House 51 for the morning. When Casey later makes a risky move in an attempt to save a distraught man from jumping off a building, Mullins has a surprise that Casey does not expect. Mouch flies in Cruz's brother Leon as a birthday surprise, but the rest of the squad struggles to keep it a secret. Meanwhile, Brett is caught off guard when she learns new information about her hometown friend Hope.

After photos of Casey and Severide jumping off the roof during a heroic rescue make the front page of the newspaper, Boden unwillingly agrees for the photographer to chronicle a day-in-the-life story at the Firehouse. Dawson and Brett are on a mission to find out what keeps happening to Gerald, a frequent flier they know who lives on the streets. Meanwhile, Herrmann decides to take up life-coaching as a side job and Kidd makes a decision regarding her living arrangement with Severide.