A blast from the past asks Mouch and Herrmann for a favor.
Meanwhile, Severide continues looking into his arson case.
Ritter lends a helping hand to a young police officer who witnessed the death of a truck driver in a fiery crash.
Kidd searches for the right person to fill the open spot on Truck 81.
When they return to the Windy City for their friends' wedding, it'll force them to take stock on where their own relationship stands.
So, what can viewers expect from Casey's return? Earlier in the show, Casey agrees to be the best man for Severide another firefighter at his wedding.