When the team responds to a semi-truck leaking hydrochloric acid in a busy intersection, Casey and Dawson find their relationship being put to the test. Meanwhile, Severide learns more about Brittany's past and Mouch has some difficulties in his dating life.

The firehouse receives unsettling news when two of their own come up missing following a routine call, prompting members of Chicago P.D. to lead the search. Meanwhile, Casey and Dawson have a hard time figuring how to juggle their personal and professional lives. Elsewhere, Chief Boden and Donna struggle with news that their newborn has potential complications and they receive a surprise out-of-town visitor.