Firehouse 51 investigates the cause of a series of mysterious gas leaks.
They find out it is an issue on the gas company's end and urge the CEO to shut off the gas main to repair the problem.
The CEO accepts the severity of the situation but is hesitant about leaving hundreds of thousands of people without heat in the winter.
The problem is later resolved and she thanks 51 for their service.
One of the calls ends with a little girl being badly burned during a gas leak explosion.
Gallo relates to the plight of the family and girl, having lost his entire family to a fire when he was a child.
He continues to visit her in the hospital and confides in Kidd the call stayed with him.
Kidd lets Gallo know he's not alone at 51.
Cruz feels the pressure of planning his wedding, and Kidd tells Severide to step it up as Cruz’s best man.
Though they get off the rocky start, Severide let's Cruz know he will be there for the groom-to-be and helps quell some of Cruz's fears about marriage.
Brett gets a letter from her birth mother and wants Casey to throw it away. She later asks for the letter back and thinks about reaching out to her birth mother who lives in Illinois.
Ritter helps Mouch prepare for an upcoming evaluation.