'Chicago Fire' Sneak Peek: Casey Gets Jealous After Catching Brett and Grainger Flirting (Exclusive)

ET exclusively debuts a first look from Wednesday's episode.

Three's company?

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..."

It's at that exact moment Casey happens to be walking by (TV dramas!) and eavesdrops on their conversation, even trying to stealthily ask Herrmann for any intel on why Grainger is even stepping foot in the firehouse.

"Psst, is that -- what's his name -- Grainger covering for you?" Casey asks with a furrowed brow, clearly a bit jealous.

Adrian S. Burrows Sr./NBC

"Yeah," Herrmann replies. By the way Casey processes the information, you know there's more he wants to say. Watch the clip above.

Recently, Spencer and Killmer talked to ET's Rachel Smith about the status of Casey and Brett's nonexistent romantic relationship following their kiss earlier in the season.

"If you can imagine it, there's going to be even more angst unfolding over the next couple of episodes," Killmer said at NBC's virtual OneChicago Day junket of their time apart. "We have some new characters coming in for both of them that are going to maybe create a little jealousy." 

"We need a bit of space, is the track," Spencer acknowledged. 

"Yeah, we're trying to take space, but the feelings for each other are very strong," Killmer noted. "So in terms of where it goes... it's been two and a half seasons of them having heat, [a] long slow burn and so hopefully they'll work it out and just get real candid. Have a real good conversation. Just let it out on the table."

But Spencer believes it's a matter of time before Casey and Brett finally get their act together in the romance department. 

"Now there's a couple more road bumps... and then I promise you, we might get together," the actor hinted. "If I were a betting man, I would say by season's end."

Chicago Fire airs Wednesdays at 9 p.m. ET/PT on NBC. For more, watch below.

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