When Burgess realises that her sister Nicole never made it home after spending the night out with friends, the team makes it a top priority to find her.
As Voight and his team attempt to track down on the bombers, they come to the realisation that there may be more bombs yet to come.
When a nighttime photographer is murdered over a piece of mysterious footage, the investigation leads to one of Chicago's wealthiest families; Burgess' sister Nicole visits the district and shows an icy attitude toward Ruzek.
Firehouse 51 joins forces with the Chicago Police Department to combat a massive explosion that leaves the city in chaos and one of the crew missing.
A hate crime tests Laurie's resolve, Kevin turns down an offer of assistance, Matt brings his pulpit to the street, and Meg takes on a new role.