Jesse Spencer

After nearly two seasons of Chicago Fire, you'd think the firefighters at Firehouse 51 had seen it all. But nothing could have prepared them for the events of Tuesday's new episode (10/9c, NBC).

In the hour, titled "A Dark Day," the Chicago hospital where Casey (Jesse Spencer) and Dawson (Monica Raymund) are volunteering for a charity run is rocked by a huge explosion. In this exclusive sneak peek, Casey quickly jumps into action as the chaos — including fires, debris and tons of injured civilians — ensues.

Scoop on Chicago Fire and more must-watch finales

But if Casey is already at the site, who will take his place as lieutenant when 51 gets called to the scene? Watch this exclusive sneak peek to find out:

Check out Chicago Fire on Tuesday at 10/9c, and Chicago P.D. on Wednesday at 10/9c for this special two-part event. Watch full episodes of Chicago Fire here.