Dennis Williams Grimes

Atwater reunites with an old friend from his academy days after a bombing rocks downtown Chicago.

Atwater (LaRoyce Hawkins) and Burgess (Marina Squerciati) are given a new patrol car, but are challenged with keeping it brand new. A run in with a psychotic woman leads the Intelligence Unit to discover a drug smuggling operation which has put several girl's lives in peril. Voight's (Jason Beghe) overprotective act with Lindsay (Sophia Bush) leads her right where he doesn't want as she also helps to keep Justin out of trouble. Meanwhile on stakeout, things get tense between Olinsky (Elias Koteas) and Ruzek (Patrick Flueger) as Olinski tries to teach him the finer points of surveillance.

Casey (Jesse Spencer) continues his run for alderman, but a citywide propaganda attack against his campaign makes him reconsider his candidacy. The rescue squad is summoned to the scene of a man trapped inside of a burning car with suspicious evidence suggesting that the fire may have been intentionally set. Mouch's (Christian Stolte) upcoming bachelor party looms, but with Trudy's (guest star Amy Morton) nerdy brother Larry (guest star Zack Pearlman) handling the details, everyone fears for the worst. Meanwhile, Jimmy (Steven R. McQueen) and Brett (Kara Killmer) take a kidnapping investigation into their own hands.

A major security breach in the city's computer networks forces Firehouse 51's senior leaders to take matters into their own hands. Severide gears up for an arson investigation.