"Three Bells"

Severide and Dawson are still out to prove the fire that killed their dear friend was due to arson. Last episode we saw, Severide found a fuse at the scene of Shay's death. But when they present their evidence to Captain Cunningham, she is not impressed. She promises to do her best with what they have, but Severide and Dawson will have to sit tight for now.

At Cruz and Otis's apartment, Otis's grandma shows up and Cruz is shocked to hear that she will be staying with them, he obviously isn't happy.

This isn't the best news to welcome Shay's sister Meghan with, who Dawson called to come to town to get more information on the death of her sister. Severide has no answers for her, until Boden calls him into his office to tell him he a phone call from Hadley in prison. Hadley tells Severide that he knows the fire that killed Shay was arson. 

Kevin Hadley is back and still psychotic! He has Severide come pay him a visit in lockup and shows him a piece of anonymous mail he recently received; a newspaper clipping of Shay's death, with a message written on it: "They shouldn't have used the word accident. -An admirer." Hadley also has the envelope with a return address on it, but threatens to hold the integral piece of evidence unless Severide could get the warden to give him a special mattress for his chronic pain.

Brett arrives at the firehouse and Dawson introduces her to Meghan. Meghan's hotel reservations fall through, Shay and Dawson invite her to stay with them. Meghan then asks Dawson why she isn't with Casey anymore.  

A call comes in and the team rushes to the fire at an office building. Inside there is a security booth with bulletproof windows and the door is locked, they see a man passed out inside and the room is full of smoke. They get to work trying to drill holes in the bulletproof glass so they can get the man out. They get enough of the glass out so that Casey can get inside. He finds the victim and drags him to the door, the ceiling caves in and Severide has to set the man down and try to make a hole in the rubble. He finally gets through, and the man is dragged outside and put on a stretcher. Brett tends to Casey, and checks his breathing. Dawson walks past him and says, "Damn baby don't scare me like that." Casey is obviously confused, seeing as how they are broken up.

At work Cruz corner Otis and wants to know when his grandma is leaving because he wants to have Brett come over. Otis breaks the news to him that his grandma is not leaving, she is actually moving in with them. A delivery man drops off an envelope at the fire house for Severide, says it is from the warden. Severide looks up the address, and it is just a vacant lot. He tells Boden he thinks that Hadley is messing with them and shows him the envelope. When Boden takes a look at the envelope, he notices the vacant lot is the same site as the fire that killed Henry Mills and where Boden got burnt himself! Meaning the arsonist has been on the loose for 20 years and has killed multiple firefighters.

While Severide is on the case, Boden takes Mills aside and fills him in on his father's involvement.

Later, when out on an ambulance call with Brett and Megan, who tagged along to see what Shay did for a living, Mills takes that aggression out on an unstable victim, not hurt but shouting for the police, saying that someone should call the police before he hurts someone. But when Brett goes to talk to him, he pulls a knife on her. Mills called the police then tackles him to the ground.

Megan is upset, realizing what her sister went through on a daily basis, and feeling guilty for not knowing her better. Mills is reassuring, pointing out that Shay never wanted people to worry about her; that's just who she was. 

Meanwhile, Severide and Cunningham are making major progress in the hunt for the arsonist. They've narrowed their search down to one suspect: Adrian Gish. The two head to Gish's last place of employment in hopes of asking him to come in for some questioning. But Gish finds them first. He goads Severide with questions about firefighters, dropping enough hints so that it's clear he is the arsonist they're after, but not enough so they could do something about it. When Gish says Shay's name, it takes every ounce of restraint Severide has not to go after him. Gish takes off, and Cunningham promises to take their findings to her superiors. 

At the fire house, they surprise Meghan with a ceremony outside honoring Shay. Boden speaks about Leslie Shay, and says there are no words that could fill the hole that she left in their hearts. Dawson joins Boden and says a few words about Shay. Afterwards the crew heads out for drinks, and Dawson and Casey sit down to discuss their relationship. Dawson explains that she wanted to be a candidate so badly, and apologizes for the way things turned out. Casey says that he is happy she is on his team, so that they can look out for each other. Dawson says she wants them to put things on hold for a while, and down the road they can see where they stand, Casey agrees. Brett heads over to Cruz’s apartment for their date night, and she brought along a bottle of tequila. Brett and Cruz start making out on the couch, and then they realize that Otis’s grandmother is sitting in the chair knitting and watching them.

The arson inspector arrives at the bar and she has bad news. The department has decided not to press charges against Adrian Gish because they don’t have enough evidence. But, the case is still open and Gish is on their radar, Boden and his team are disgusted that Adrian is just going to walk.
The episode ends with Dawson calling her brother asking for his help.