Discovering the KGB is one-step ahead, Joe tries to convince the team to run a double agent scheme - a perilous game that understandably makes his colleagues uneasy. But with dangerous assassin Odin at large, the team is unsure KGB defector, Arkady, would risk his life to help them. Meanwhile, Alan grapples with a shocking discovery that could jeopardize his marriage to fellow agent and wife, Sarah.

The team is reeling from a KGB bombing and the mole's confession, but with Operation Glass hurtling towards a shocking climax, MI5 is running out of time to piece the puzzle together and avert disaster. Bobby and Wendy make a crucial discovery, which could have implications at the highest levels of government.