Harry is framed when an asset to a top-secret operation known as Sugarhorse is found dead. Harry contacts Lucas for help before he is arrested and taken into MI5 for brutal interrogation by the Director-General Richard Dolby.

Meanwhile, Lucas travels to Russia to meet with one of Harry’s trusted contacts, Maria, who has intelligence that can help protect Britain from nuclear attack. With the Russians closing in Lucas finds the location where the information has been hidden and is horrified to uncover the identity of the mole.

Back on the grid, Dolby informs Ros of Harry’s arrest and tells her to investigate the death of the Sugarhorse asset in order to prove Harry’s innocence.

Ben notices an inconsistency with the Sugarhorse asset’s death and visits the archive to dig deeper. When Lucas calls Ben to reveal the traitor’s identity, Ben doesn’t realise just how close the mole is…