Halloween is usually a time costumed fun. But for the firefighters of Firehouse 51, it’s one of the most nightmarish times of the year, as foolish pranks go wrong and turn into unpredictable emergencies. Keeping with that annual tradition, Wednesday’s Chicago Fire will see Truck 81 take on a seemingly normal call but things quickly escalate when they realize the situation is far worse than they anticipated.

In an exclusive preview of this week’s episode, titled “All the Proof,” Truck 81 responds to a call about a potential gas leak at a busy hotel. As they inspect a floor filled with hazy smoke, Severide (Taylor Kinney) and Casey (Jesse Spencer) make a startling discovery: A couple leaked chemicals into an air vent in what appears to be a suicide pact. With dangerous fumes freely flowing in a building full of unsuspecting guests, the crew faces a mountain of a problems exacerbated by the fact that so many lives are at stake. Can they get everyone out alive?

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The episode will also see Gorsch’s (Steven Boyer) reign of terror continue as he takes over Boden’s (Eamonn Walker) morning meeting and goes out on calls in the chief’s place, fanning the flames of an already heated feud. Plus, Brett (Kara Killmer) and Foster (Annie Ilonzeh) respond to a call where a Halloween prank turns deadly serious.

See how it all goes down when Chicago Fire airs Wednesday at 9/8c on NBC.

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Taylor Kinney, <em>Chicago Fire</em>Taylor Kinney, Chicago Fire