Firehouse 51 will attempt a daring rescue when one firefighter goes missing in an explosion on the next episode of "Chicago Fire."

On the next episode, "A Dark Day," Chicago Fire will team up with the police from "Chicago PD" when an explosion occurs at Chicago Medical, where Casey and Dawson happen to be volunteering.

The synopsis for the episode reads: "A large explosion sends Chicago into chaos; the members of the firehouse are tested when one of their own goes missing in the blast."

The crossover special will be a two-episode event. 

On the last episode before show's week-long hiatus, Boden was told a life-changing secret and Severide helped a firefighter in need.

In "A Heavy Weight," Dawson (Monica Raymund) doesn't know how to handle receiving a suicide note from Jones. She receives the note after Firehouse 51 finds out that Clarke (Jeff Hephner) transferred to House 17 without a goodbye or an explanation.

Afterwards, Severide (Taylor Kinney), decides to check on Bloom, because he finds out that Bloom checked out 12 hours into his 28 day stay in a rehab facility.

Meanwhile, Casey (Jesse Spencer) decides to pick out an engagement ring for Dawson, but he has one issue -- he doesn't know her ring size. Later, Shay (Lauren German) gets Dawson's ring size for him by estimating with a piece of newspaper.

Later, the gang receives a call about a fire at a gas station. They rush to the scene and save a woman whose husband drove their vehicle away with the gas still pumping.

After the daring rescue, Severide decides to help Bloom, who is depressed due to an incident in which some firefighters on his team died during a rescue, and he wouldn't let any of the other firefighters try and save them as he knew it would mean certain death. His firefighters held the controversial decision against him, which Severide says is a "heavy weight" to carry.

Severide goes to Bloom's trailer to visit him, but Bloom escapes, leaving the stove on.

The team then finds out that Clarke transferred because he made Lieutenant. It turns out he's working with Jones' oldest brother.

Boden then goes to see Donna to profess his love to her, but she brushes off his romantic pleas.

The team then gets another call about a car accident that involves a bus of kids on their way to prom, and one of the kids got ejected from the vehicle.

The police refuse to stop traffic, so Boden has some of the firefighters drive a truck into the road to put a stop to traffic. The police arrest Boden, and Casey threateningly asks the officer if he "really wants to die" on that road.

After they rescue the high school kids, Severide arranges to have two of Bloom's men come to Chicago from Denver. They tell him that they now believe that Bloom made the right call, because they now realize that they also would have died in the building fire if they had gone in to save their fallen colleagues.

Later, Donna shows up at the firehouse, revealing that she's pregnant with Boden's baby.

Later in the season, executive producer Matt Olmstead hinted that Dawson and Casey may be headed to the altar. However, the impending engagement may come at a bad time for Dawson.

"That impulse [to settle down] is coming at the wrong time really with Dawson," Olmstead explained. "When she decided to become a firefighter, it all came together for her in terms of, 'Wow, this is really what I wanted to do all along without even knowing it.' It's definitely a conflict between the two of them, and it's a bit heartbreaking because they do love each other. But are they going to be able to overcome this obstacle?"

Watch the promo for "A Dark Day" below, which airs Tuesday, April 29 at 10 p.m. EDT on NBC.