A car explodes during a chase; the registered owner is a Marine lance corporal; while Gibbs and company investigate, McGee finds radioactivity in the trunk. The gang catch a clue about the whereabouts of the brother of the owner, and they find him, then they run into "MAllison" Hart and two of Col. Bell's bodyguards. Tony and Ziva check out a vacant warehouse and find some radioactive evidence; Ducky concludes that the victim is not the owner of the car; McGee learns that the victim is a former sergeant who was dishonorably discharged. Tony and Ziva find the owner and deliver him to the chat room, where Gibbs questions him. McGee and Ziva talk with several dentists, one of whom takes a long lunch break. TV news speculates about dirty bombs, so the people of Washington shift into the panic mode; Gibbs and McGee seek a bomb, find it, and disarm it. Vance and the gang figure it out and wind it up. Gibbs receives a visitor at his home.