So, we get both the aftermath of the 9-1-1 call that we left off with last episode, and the beginnings of Karen and Hen's relationship.
OMG. Chim set them up?! Also, present day Karen seems so much calmer and laid back than previous Karen. Love that for her.
Anyway, we get the explosion in the lab, and it turns out Denny is there. It's Take Your Child to Work Day. Thankfully, he gets out almost immediately and physically, is completely fine. However, he's on the phone with Hen when the explosion happens and she rushes over immediately. However, Bobby tells her that she can't help because she's not a member of the 118 anymore (but she didn't turn in her resignation papers yet), but also because she's too close to the situation, as Karen is trapped inside.
Inside the building, we look for survivors and put out fires. We find Karen pretty quickly, and she seems okay, so they ask her to assist while they help some other people. Once that's done. Buck goes to escort her out of the building. But before she makes it out, someone escapes from the building and has a message. Apparently, the Clean Room will explode if the door to it is opened. Karen helps direct Bobby there just as another captain is opening the door. Bobby manages to pull him out of the way, but the room still blows up.
Outside, Hen is busy doing some kind surgery on one of the victims. It looks intricate, but she seems to have it under control. When she finishes up, Karen finally comes out. She seems a little shaky, but okay. Until she's not. Turns out she got hit by some shrapnel and didn't notice. She collapses and her heart stops. It takes several attempts, but they finally save her. Turns out, she had ruptured her spleen.
Ultimately, at the end of the episode, Hen decides to give up med school, and continue being a paramedic. Turns out over the years, her dream has morphed, just like Karen's did. And they both claim they are currently living their dream. It's very sweet.