A new chief shakes things up at Firehouse 51. Violet and Novak work to uncover the truth behind a car crash. Cruz keeps a watchful eye on Severide and Damon's growing bond.

After Damon disobeys Kidd's orders on a call, he turns to Severide to protect him. Herrmann searches for the perfect scent to spice things up with Cindy.

Severide and Van Meter investigate a fire at a family-owned restaurant. Pascal questions Kidd's leadership.

Violet risks her job to deliver a baby on a tragic call. Mouch, Capp and Tony help Cruz track down Javi's stolen bike. Damon's departure weighs on Severide.

Kidd and Severide investigate a trucking company responsible for a string of accidents. Herrmann recruits Kylie to help get new radios. Firehouse 51 celebrates Violet's birthday. Carver's allegiances are torn.

Severide gets wrapped up in an Illinois State Police investigation after discovering illegal guns on a call. A bird harasses Mouch on the apparatus floor. Kidd keeps an eye on Carver as his attitude worsens.

Pascal and Severide revisit the storage facility fire in hopes of uncovering new evidence. Cruz helps Mouch take his studying to the next level. Novak prepares a proposal for Chief Robinson for a whole blood program.

Herrmann and Mouch prepare to take their officer exams. Cruz receives a chilling omen. Carver takes care of a dog after its owner is injured on a call.

Cruz's past comes back to haunt him. Severide prepares to teach an arson class at the academy. Mouch searches for a victim's next of kin.

The team responds to a crisis at a community center. Severide is forced to investigate one of his own squad members. Kidd helps a teacher going through a post-incident spin-out.

This is the first part of a major crossover with "Chicago Fire," "Chicago Med," and "Chicago P.D.": Chicago Fire – S13E11 – In the Trenches (1) Chicago Med – S10E11 – In the Trenches (2) Chicago P.D. – S12E11 – In the Trenches (3) ************************************* Chicago's first responders unite in the wake of a catastrophic gas explosion and fire that threaten a city government building. Herrmann reluctantly shadows Pascal while Violet, Novak and Dr. Frost take charge of triage efforts on site.

Herrmann investigates the rise in missing fire hydrants and teams up with Kylie to track them down. Kidd's cousin comes to town, revealing surprising family history. Violet and Novak treat the victim of a birthday cake smash gone awry.

After a daring rescue at an adventure park, Kidd asks Severide to create a ropes course for Girls on Fire. Violet's former nemesis returns, pleading for her help. Herrmann creates a signature cocktail for Molly's.

A surprise visitor from Novak's past joins Ambo for a late-night ride-along. Herrmann faces a mountain of Chief-level training records and incident reports. Violet catches Carver in a lie.

Kidd and Severide eagerly await big news. Pascal searches for the perfect anniversary gift.

In the face of adversity, Pascal searches for justice. Violet struggles to write a letter to Carver. Damon returns to the floater pool hoping to secure a permanent spot at 51.

Kidd connects with a troubled teen interested in Girls on Fire. Cruz, Capp and Tony eagerly wait to try out Squad's new saw. Violet and Novak are stumped by a bodybuilder who passed out at the gym.

Boden returns, having been tasked by Commissioner Grissom to run a post-incident analysis following a house fire that collapsed with a firefighter inside.