On Wednesday's Chicago Fire, titled "Escape Route," Sylvie Brett (Kara Killmer) begins to move on from Matthew Casey (Jesse Spencer) when Greg Grainger (Jon Ecker), who's taken a liking to her, subs in at Firehouse 51 as Christopher Herrmann (David Eigenberg) gets ready to go on vacation. Grainger's appearance around the firehouse ruffles some feathers, of course -- specifically Casey's. In ET's exclusive sneak peek of the episode, Casey catches Brett and Grainger flirting by the lockers and is clearly unsettled by the new arrival. "Sorry, lieutenant, this locker room is for 51 firefighters only," Brett says with a twinkle. "Well, I picked up a double," Grainger reveals. "And as luck would have it..."