Tracy Spiridakos

The team investigate a drug deal gone bad after the body of a college student is found; Detective Hailey Upton temporarily joins the team and finds her investigative methods clash with the Bureau's more buttoned-up environment.

Kidd and Severide's honeymoon is interrupted by a dangerous person from their past; Kidd's former academy classmate joins Firehouse 51; Brett and Violet reflect on their relationships.

Mouch builds a classic fire truck model for the deputy district chief but needs assistance from Gallo and Ritter; Brett and Violet encounter multiple victims with a strange set of symptoms; Cindy's chemotherapy results loom over the Herrmann family.

A spate of robberies comes to light after a firehouse lock box key goes missing. Sgt Hank Voight suspects a firefighter may be involved, and with Boden's support, he recruits Cruz to go undercover and sniff out any suspicious activity.

Mac joins forces with special agent Jack Dalton to recover a missing bioweapon and save hundreds of thousands of lives.

Tensions run high between Casey and Herrmann after the firehouse responds to an apartment complex fire. After discovering something extremely suspicious during the call, Kidd and Severide do some investigating and get CPD on the case.