Joan gets a call. Her mother, who has Alzheimer's, is missing. Marcus sees a suspicious bag at the precinct, and it starts to smoke. He throws it into an interrogation room and seals it inside. It's a biological attack. Marcus is exposed and isolates himself. Marcus thinks whoever set it off is still in the building. While Joan is calling around, her mother shows up. Sherlock hears about the quarantine and mentions to Gregson the Leon cartel used an anthrax attack against a Mexican police station. The CDC arrives. Gregson asks the CDC's Dr. Tanaka to decontaminate a cell phone attached to the device. Sherlock and Joan go to talk to the cartel leader in New York. He said he had nothing to do with the attack. Marcus is decontaminated, but another man exposed shows symptoms. Gregson and Marcus interview all visitors. Sherlock builds a model of the precinct, to figure out when it was scouted. Tanaka figures out the device came in two pieces and was built right before it went off. Sherlock determined the date, and only cops were on the floor that night for a union meeting. Sherlock picks out Sean O'Grady as a suspect. The police computer system goes down. Gregson discovers all the servers have disappeared, carried out by someone in a CDC hazmat suit. The data on them would be worth millions. The smoke was just a distraction. Marcus thinks the two men who had symptoms are in on the heist, as was O'Grady. All three were bankrupt, linked by the same repo man, who stole the servers. All had bought plane tickets to escape the country. The repo man was already arrested. Joan confronts her mother about hiring a professional nurse because of her Alzheimer's, and her mother agrees.