Chicago is in turmoil over constant terror threats, and it’s up to Engine 51 to protect the city.

Engine 51 gets a call to Soldier Field, where mysterious purple smoke is escaping from the premises. Lieutenant Kelly Severide (Taylor Kinney) goes in and finds a smoke grenade along with many others that were left inside. Agent Alex Ward (Liza Bennett) and police assist on scene to investigate.

An argument arises at Soldier Field between Paramedic Chili Chilton (Dora Madison) and Paramedic Sylvie Brett (Kara Killmer) while caring for a patient. This argument is just the beginning for these two. Brett later confronts Chilton about her drinking problem. In return, Chilton walks directly into Chief Boden’s office and requests for a new partner.

Another bomb threat and multiple hoaxes later, terror spreads throughout the city. Ward believes that someone is preparing to attack.

Meanwhile Firefighter Christopher Herrmann (David Eigenberg) prepares Candidate Jimmy Borelli (Steven R. McQueen) for a Battle of the Badges boxing match that will be held by Firefighter Randy “Mouch” McHolland (Christian Stolte), between “the heroes of 51 and those blockheads down at the 21st police district.” During practice, Firefighter Gabriela Dawson (Monica Raymund) reveals her “silver gloves” undefeated boxing history from high school and hits a hard punch knocking tough-talking Herrmann right to the ground.

Lieutenant Matthew Casey believes Alderman Colin Becks is getting a financial cut from his partnering charity. Lt. Casey confronts Becks and even discusses over dinner that he should be donating towards the tornado victims. Becks seems to bribe Casey with a trip to his lake house. Conveniently minutes later, Becks gets a phone call and has to leave.

Engine 51 receives a call to a seemingly empty building. They find out 15 people are inside and work on leading them outside through the exit until it becomes an active shooter situation. Severide spies a guy on the roof of the building who starts shooting at hostages as they exit. Paramedics take cover behind a couch while they hear gunshots fill the air. Truck 81 hides inside a room with hostages while shooters stalk the grounds.

Severide tries to rescue hostages by ladder and while Ward protects him the shooter reappears. Ward shoots but is shot in return and is taken away in an ambulance. As S.W.A.T. arrives, Casey leads his team and hostages towards any possible exit once smoke appears in their hideout room.

The paramedics hear a female cry of pain in the distance and decide to go aid her. When trying to heal her, footsteps head their way. They look up to see the shooter with the gun aimed right towards them. Before he gets the chance to shoot, the S.W.A.T. team shoots him right in the back, ending the active shooter situation with only 4 dead and 7 wounded.

Severide goes to visit Ward in the hospital. Once he steps out into the hall he overhears that she will be relocating to Miami. The love interest may be coming to an end as Ward gets up and chases after Severide to tell him, “I’ll never forget you.”

Firefigher “Mouch” and wife, Trudy, made the decision to donate the funds from Battle of the Badges to “the families who lost something today.”

“There’s always hope as long as there’s good people in this world,” Chief Boden reminded his firefighters at Molly’s.

Catch the next episode of Chicago Fire on Tuesdays at 10/9c on NBC.