S1, E2: The day after Litvinenko's death, radiation teams swarm across London to secure potentially contaminated sites visited by the Russian spy. Officers discover traces of polonium at a sushi restaurant where Litvinenko met Mario Scaramella before a later encounter with Andrey Lugovoy and Dmitry Kovtun at the Millennium Hotel, which also tests positive for polonium. DS Clive Timmons realizes Lugovoy and Kovtun are their main suspects and resolves to send a team to Moscow to interview them directly.