Season 4 Episode 15
"Bad for the Soul"
51 welcomes a new firefighter; Borrelli joins Brett as a paramedic; and an accident at a neighboring firehouse causes suspicion.
Squad 3 is called to a neighboring firehouse to help assist after an accident. Cruz (Joe Minoso) immediately notices that the events surrounding the accident are suspicious, and Severide (Taylor Kinney) files his report detailing Cruz's observations. After further investigation, it's revealed that a firefighter at the neighboring firehouse lied about the accident.
Borrelli (Steven R. McQueen) is crushed when he learns that he will no longer be able to be on the truck, as a firefighter with more seniority will be taking his place. Later, he's encouraged when he discovers that his paramedic training will enable him to join Brett (Kara Killmer) as her new partner.
Casey (Jesse Spencer) begins his campaign for alderman in earnest and attracts several powerful Chicago backers. In his first foray into the dangerous side of politics, he's approached by a group of "concerned citizens" - who turn out to be leaders of Chicago's most dangerous gangs.
Firehouse 51 welcomes a new firefighter into the family with Stella Kidd (Miranda Rae Mayo). Kidd is friends with Dawson (Monica Raymund) and also has a history with Severide, with whom she shared a one-night stand.