S6, E13: While working to track down a bombing suspect, Voight (guest star Jason Beghe) and Antonio (guest star Jon Seda) meet with Chief Boden (Eamonn Walker), who unwillingly agrees to let Dawson (Monica Raymund) and Brett (Kara Killmer) go undercover. As the investigation continues, Boden, Casey (Jesse Spencer) and Severide (Taylor Kinney) lend their skilled expertise in the investigation. Elsewhere, Kidd (Miranda Rae Mayo) and Zach's (guest star Daniel Di Tomasso) relationship hits a bump and Cruz (Joe Minoso) has a hard time expressing his true feelings for Brett. Meanwhile, Herrmann (David Eigenberg) and Connie (guest star DuShon Monique Brown) are forced to come up with an alternative solution when two birthday parties are booked on the same day at the firehouse.