While the squad is still reeling from Hallie's death, Herrmann announces that the grand opening of Molly's will now be a memorial service for Hallie. Chief Boden also announces that arson investigators are still analyzing the scene of Hallie's death and that Casey is meeting with the Chicago Police Department to get an investigation rolling. It's a tough day at Firehouse 51.
Meanwhile, Casey heads to the police department to meet with the detectives who are investigating Hallie's death. Voight's team is on the case, but Casey says he doesn't want to deal with anyone but Antonio. Antonio tells Casey that initial investigations suggest that Hallie was killed before the fire was started. Suddenly, a call comes through on the radio.
CPD has tracked down the car that fled the clinic fire and they have two suspects fleeing on foot. Two patrol officers give chase and catch their suspects, two drug addicts who bought the car from an unknown seller recently. The CPD's lead detective knows that the vehicle is used to traffic large amounts of drugs. Their murderer is in the drug trade.
Boden, Casey and Severide head back to the scene of the clinic fire to investigate with the Chicago police. Casey still doesn't trust Voight, but so far Voight seems to be sympathetic to Casey's loss. Inside the ruins of the clinic, Boden notices that the clinic's patient files have been rummaged through. Before they can continue the investigation, Boden and Severide are called in to help a woman who is trapped in a sewer drain.
Down in the sewer, Severide finds the trapped woman and he and Boden stage a rescue. They manage to dislodge the woman from the suction of a sewer drain and pull her to safety. Meanwhile, Antonio and his fellow detective question the clinic owner for possible leads in the investigation of Hallie's death. The owner tells them that someone had threatened him in order to obtain prescription drugs.
Back at the station, Otis brings his Russian cousin in to help with the grand opening of Molly's. Dawson and Herrmann are worried that Otis is going to spoil the event, but Otis assures them that everything will be fine. Voight and Antonio relay the drug information to Casey, who says that, as far as he's concerned, Voight is the prime suspect in Hallie's murder. Antonio assures Casey that he'll handle it.
Shay and Dawson are called to the scene of a gunshot accident where a nurse has been shot in the stomach. The victim is dead when they arrive on the scene, but Dawson discovers that the nurse worked at Hallie's clinic. Someone's taking out clinic nurses one-by-one. Meanwhile, Shay and Severide head to the clinic for Shay's fertilization procedure and Dawson, Otis and Herrmann continue to prep the bar for their grand opening.
Dawson meets with Mills at a cafe. Their relationship has been cold ever since Dawson broke the news to Mills about Boden and his mother. During their meeting, Mills tells Dawson that he needs some time apart. Just like that, Dawson and Mills break up. Meanwhile, Voight learns from his street contacts that a local drug dealer was procuring prescription pills from the clinic owner.
Severide consoles Casey by offering him a cigar. Outside, Casey vents to his old friend and Severide does his best to put his friend at ease. Antonio stops by the station to solicit Casey's help in setting up a sting operation against the clinic owner. Casey needs to tell the owner that he knows he's been dealing prescription pills on the side and that he needs to know the name of the owner's drug connection. Casey agrees to the operation.
Later, Casey enters the clinic with an army of police officers surrounding the building. Before Casey can get into the owner's office, gunshots ring out. The clinic owner has been killed by his drug connection. Casey chases the shooter onto a train. At the next train stop, the suspect holds Casey at gunpoint. Voight shoots the gunman in the head, killing him instantly and saving Casey's life.
That evening, Voight meets with a mysterious official who tells Voight to keep pretending he's a dirty cop so that they can catch some of the city's most ruthless drug kingpins. Meanwhile, the Firehouse 51 crew meets at Molly's for the bar's grand opening. At the gathering, Casey shares some of his fondest memories of Hallie. Everyone is heartbroken, and they need each other now more than ever before.
Casey teams up with the Chicago Police to investigate Hallie's murder.