10/08/2013 (10:01PM - 11:00PM) (Tuesday) : SEVERIDE IS CONVINCED HE KNOWS THE ARSONIST'S IDENTITY-MENA SUVARI GUEST STARS-Lt. Severide (Taylor Kinney) has his strong ideas on who the serial arsonist is, but first needs to find proof to back up his suspicions. Meanwhile, Lt. Casey (Jesse Spencer) adjusts to his new lifestyle in the wake of recent events and Molly's bar is failing, much to the dismay of owners Hermann (David Eigenberg), Dawson (Monica Raymund) and Otis (Yuri Sardarov). Elsewhere, Dawson goes out on a date with Jay Halstead (guest star Jesse Lee Soffer), one of the bar regulars, and Mouch's (Christian Stolte) campaign trail starts to get on track with the aid of Isabella (guest star Mena Suvari), an old friend of Dawson's. Eamonn Walker, Lauren German, Charlie Barnett and Joe Minoso also star. Treat Williams guest stars.