Chief Boden is with Donna as she gets an ultrasound and they agree to wait, for now, to find out the sex of their baby. The doctor then shares with them the bad news that Donna has a condition called placenta previa. It puts both her and the baby at risk in the 3rd trimester.
Brittany wants her new husband Severide to call in sick, but he can't. She asks him to promise her that he'll be safe, and he says he'll be fine.
Casey sees Cruz and Brett talking in hushed whispers about Zumba and assumes they are getting closer on a personal level.
The squad is called to the scene of a downed helicopter that has exploded on the roof of an apartment building.
Brett asks Mills about keeping a secret with someone in the firehouse for personal reasons. He warns her that in his experience in house relationships are better kept out in the open.
Severide tends to the pilot and 2 passengers still stuck in the downed helicopter that is dangerously balancing on the roof as Mills and Brett help victims on the ground. Meanwhile, Otis and Herrmann try to help a woman pinned under the rotor.
Brittany shows up to the scene worried about Severide, having heard his voice on the radio. Boden tells her she can't be here and moves her behind the line and tells her to Severide do his job.
Mouch and Cruz come out of the apartment building but there's no sign of Dawson. As Casey heads into the building there's an explosion from within. He finds Dawson, and while she needs a once over from the paramedics, she'll be ok.
Herrmann and Otis free the woman pinned by the rotor, while Dawson has found a blood trail for a possible victim that she's going to follow.
Mills and Brett finally reach a victim trapped by the helicopter's tail piece. When they get into his van to free him, they notice a bag full of passports.
Otis discovers a remote controlled drone stuck in the rotor.
Dawson finds that the blood trail leads to a teenager with a head wound, clutching a remote controller and apologizing profusely, saying that he screwed up.
Mills leave Brett at the van in order to go help Dawson. The man in the van wakes up and starts freaking out. He pulls a gun on Brett and tells her he's not going to the hospital.
Dawson's teenage victim says that his brother was with him when the helicopter went down. Casey finds the boy unconscious in a high voltage area near a transformer. He's able to rescue him before an electrical fire happens.
Severide frees the pilot of the helicopter, but the two passengers are still pinned inside. As the helicopter precariously balances on the roof, Severide works to free them. He is able to get them out and get himself to safety.
Dawson helps Mills load the boy onto the ambulance and the two share a laugh, saying it feels like old times.
Someone asks Severide if his wife has found him yet and he sees he has 37 unread messages on his phone.
Brett returns to the ambulance to ride with Mills and says that her patient refused treatment. She then guesses correctly that Mills had a relationship with Dawson. He agrees to explain it some time over beer.
Casey wants to get a hotel room for him and Dawson to celebrate their big saves. She correctly guesses that he wants a hotel because he didn't tell Severide to leave yet.
Cruz asks Brett if she's doing ok and gives her a "Free Class" pass for Zumba. Casey sees the moment and confronts Brett about what he thinks is a romance happening. Brett ends up telling Cruz's zumba secret to Casey and he immediately tells Dawson, and so on and so on ...
Herrmann talks to Boden and reminds him to stay positive about Donna's condition.
As Brett drives away from the station she's followed by an unknown car. She's afraid, worried the man from the van wants to harm her, but turns out he just wanted to thank her.
Boden meets up with Donna at a Doctor's appointment and she shares the news that they are going to be having a boy.
Severide and Brittany fight over her showing up at the scene. Dawson and Casey think it may be the end of their relationship, but then overhear the two making up.
Everyone shows up for Cruz's Zumba class. His secret is out!
Brittany and Severide talk and she is still clearly hurt over losing her sister to an accident. She apologizes for showing up to the scene, and he reminds her that he's not going anywhere.