Season 4 Episode 18
"On the Warpath"
Brett becomes a gunman's target after witnessing a murder; Mouch and Platt tie the knot in a ceremony at Molly's.
Brett (Kara Killmer) fears for her life after she witnesses a murder and becomes the gunman's next target. After investigating the shooter, Chicago P.D.'s Antonio (Jon Seda) helps Brett receive around-the-clock protection, and Boden (Eamonn Walker) keeps Brett inside the safety of the firehouse during her shift. Later, in an ironic twist, Borrelli (Steven R. McQueen) and Dawson (Monica Raymund) are called to the scene of a fatal gunshot victim... who also happens to be Brett's gunman.
Meanwhile, Casey's (Jesse Spencer) first test as alderman ends in bitter defeat when he is unable to stop a negligent contractor from receiving a lucrative city contract. Mouch (Christian Stolte), after sweating out his marriage vows and thinking up ways to postpone the wedding, ties the knot with Platt (Chicago P.D.'s Amy Morton) in a ceremony at Molly's.