A serious explosion mars a charity run attended by Casey and Dawson. 

Hermann is acting Lieutenant for the day and he freaks out because he doesn't have time to prepare mentally.

Casey and Dawson are helping prepare for a 10k race and Burgess and her niece Zoe are helping in the same tent.

Dawson reminds Casey that Burgess helped find her nephew Diego.

Dawson goes in to register for the race and the building explodes.

Casey is knocked out, but he's OK.

The carnage is brutal. Burgess' niece is among the wounded.

There is a giant hole in the side of the hospital where the ER once was.

Rafferty is subbing for Dawson and hits the scene with Shay.

The team heads to the rubble to find survivors, including Dawson.

Shay is hit by falling debris and plunges into exposed rebar. She doesn't stop.

Another firehouse show up and when Boden tries to warn them away, he refuses.

A man who appears to be foreign approaches Casey demanding to be let out. He shows a gun before the ceiling collapses and afterward complains about someone being in protection.

Casey and Severide take off their gear to climb into the debris.

Shay is badly wounded. She puts on some gauze and tape in the bathroom so she can go back to work.

Dr. Arata helps with intake of the injured.

Hermann brings lightness to those he's trying to save.

Mills and Cruz find the second bomb in a car in the parking lot.

Burgess finds out her niece has a crushed liver and needs an immediate operation. When she says she doesn't know if she has the authority to give consent, that's good enough for Arata.

Boden goes into the car with the bomb and asks Cruz and Mills to leave the area. Not a chance.

Rafferty, Shay and the pediatrician are walking through the hall and someone gives them yellow tags to mark the victims. There is an entire room full of people waiting for attention.

Mills stops the timer on the bomb.

Burgess finds out her niece needs a liver transplant, but she can't get through to her mother or her own parents. Arata is on it.

Dawson wakes up but cannot scream. She grabs a pipe and starts pounding.

Shay looks like death warmed over and her wound has grown.

Casey finds Dawson.

Atwater shows up to support Burgess.

Arata finds an adult liver, but when he gets to it, it's warm and unusable. Burgess collapses.

The firefighters remove the remaining debris hand to hand so they can remove Dawson.

Chicago PD arrives on the scene.

The liver was for the foreign guy. Voight approaches and takes their weapons. They are diplomatic protection from the Syrian Republic.

Imogene is getting surgery on her brain.

The Fire Commissioner arrives, notes he knows they were first on the scene and saved dozens of lives today. He tells them to fall back. Firehouse 51 decides to stay.

Shay is in the hospital, and when she hears that Dawson has been saved, she collapses.