The final series opens with Harry Pearce being invited back to MI5. After being faced with charges of exchanging national secrets for his personal relationship with Ruth, he stringently denies any wrongdoing. Going back to his job on probation, he returns to discover that Erin Watts has been covering his job while he’s been away.
It soon transpires that Harry is being tracked by someone. Delving deeper, he discovers that it’s Sasha Gavrik – a Russian spy who’s recently killed former MI5 agent Max Witt. What’s more, Sasha’s father – a leading Russian minister – has been smuggled into the country with his wife without informing intelligence.
Erin tracks Mr and Mrs Gavrik, discovering that the Home Secretary is in on what’s happening. Later, Harry confronts the Secretary on his doorstep, and finds out that Britain is being lined up to form a special relationship with Russia. Harry’s told to attend a special Anglo-Russian evening the following night.
Arriving with Ruth, Harry goes inside. Everything’s seemingly fine, but they find out that there’s an assassin lurking to take down Russian minister Gavrik. Finally, Harry reveals that Sasha Gavrik is, in fact, his son.