Joan and Lin are at their biological father's funeral. They make plans for lunch the next day. Sherlock thinks that a man was killed by manmade lightning. He suspects an experimental lightning gun was used to kill the man, Rohan Giri, an inspector for the Department of Energy. Giri's wife Chloe admits she was having an affair, but she was with her lover when Rohan was killed. Lin collected their father's personal effects from the shelter, and among them one letter,  to Joan. The murder weapon is Sparky, a laser cannon. The project's financier, Dave Amberlin, admits to shooting Giri with Sparky, accidentally killing him. He also told Marcus and Sherlock that his illegally bought plutonium had been stolen. Agent Kohler, a government investigator, fears a dirty bomb. Joan found a stolen delivery van near the robbery site in traffic-camera footage. It had been recovered in New Jersey. Based on the gravel in the tires, Sherlock suggests Kohler's team looks in Bedminster, N.J. The plutonium had been on the property of a known bomb maker, but he's dead and the bomb isn't there anymore, and is likely in possession of a white-nationalist group. They need to find the group's leader, Colm Frick. They capture him after he enters a tabletop war games tournament. Frick claims to know nothing about a dirty bomb. A tip is phoned in about a taxi bomb attack on a mosque. From traffic-camera footage, Marcus sees a driver abandon a taxi outside a mosque. Sherlock decides it's instead a diamond heist, by four men disguised as cops. Gregson stops them, even after the leader bluffs that he will set off a dirty bomb. Kohler was in on the heist. His Dutch girlfriend told Marcus where to find the plutonium. In his letter, Joan's father told her all about Lin.