Captain Gregson returns to work. The squad presents him with a giant captain's shield. Detective Bree Novacek is leaving the force for the private sector. Ridley Dineen, a robber who steals from other criminals, is found dead after a big score hitting a stash house. He was killed by a potent brand of Fentanyl. His girlfriend said he never used drugs, since his brother died from an overdose. Gregson fears that Novacek is leaving because of Captain Dwyer. He asked Joan how Dwyer was around her. Someone laced Dineen's inhaler with Fentanyl, then spread cat hair around his apartment to make him use it. It was likely the same person who tipped him about the stash house. Marcus and Joan suggest it may have been someone with the Russian government. Sherlock checked with Olga, a former Russian spy, and asked her to check into Dineen. Gregson confronts Dwyer about Novacek's leaving, but he denies that he did anything. Dineen shot and killed a textile restorer, Cecil Troy. Troy tipped Dineen to the stash house. Troy and Dineen grew up together. Olga tells Sherlock the Russians are keeping an eye on an emigrant scientist who helped develop the aerated Fentanyl. Then she signals and the scientist's apartment is bombed. Bree blames the police culture for its treatment of women. She doesn't see any value in reporting Dwyer's behavior. Sherlock thinks that Dineen had been targeted for robbing fronts for the Russian mob in Brighton Beach. Joan finds a memo at the DEA that explains who is behind Dineen's murder. Cecil and his partner, Audrey Kensit, who were laundering drug money for the DEA. They set up Dineen to prove that contact with Fentanyl-laced cash could be dangerous. Audrey paid the scientist to make the Fentanyl spray. Bree turns in Dwyer.