Firehouse 51 is once again under siege! This time, however, Chicago Fire's brave firefighters won't be at the mercy of dangerous criminals. Thursday's thrilling episode will see the crew helping out the FBI with an undercover mission, and TV Guide has your first look.

In this exclusive sneak peek, Severide (Taylor Kinney), Casey (Jesse Spencer) and the rest of the gang are stunned to see an onslaught of mysterious black SUVs race into the firehouse's driveway unannounced. Confused by their sudden arrival, Boden (Eamonn Walker) immediately tries to get to the bottom of things.

"What the hell is this," he inquires in the video.

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Unable to immediately provide answers, the lead agent confirms that Boden is charge and then orders the firehouse's doors closed. With the agents and firefighters now hidden away from the rest of the world, the government official drops the first hint about their secret plan: "We need to borrow your firehouse," he tells Boden.

Thursday's episode, titled "When They See Us Coming," finds Casey and Severide volunteering to assist the FBI with their covert operation. While we don't know exactly what they'll be doing or why they are needed, it must be something major since the mission is shrouded in secrecy.

And while Severide will be preoccupied with his new undercover gig, he'll also have to deal with the fallout from last week's awkward revelation that his parents — who have not been on good terms for years — are hooking up again (in his bed, no less!). At least he'll have Kidd (Miranda Rae Mayo) to lean on to for support.

Chicago Fire airs Thursdays at 10/9c on NBC.

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