The episode was written by head writer Neil Cross; with additional writing by Ben Richards; and directed by Colm McCarthy.
However, the MI5 team discover he is overseeing a cyber attack in progress, and returned officer Lucas North (Richard Armitage) may be involved.
The episode attracted over five million viewers after its broadcast, as well as receiving generally positive reviews from television critics.
Eight hours following the death of Adam Carter, Ros Myers (Hermione Norris) and Sir Harry Pearce (Peter Firth) discuss how they are to get back at FSB head of operations in London Arkady Kachimov (Stuart Wilson).
However, Harry decides to disobey and go after Kachimov anyway, and promotes Ros to chief of Section D, a position previously held by Adam.
Later, Lucas is appointed a handler from the FSB, and is shocked to find it is his ex-wife, Elizabeta Starkova (Paloma Baeza).
Meanwhile, Malcolm Wynn-Jones (Hugh Simon) discovers a communication burst coming from the North Sea, believing a Russian submarine is entering British waters unannounced to carry out a mission.
After learning that Alexander Beletsky (Georg Nikoloff), a man with connections to Kachimov, is going to shut down his company's servers, Malcolm confirms his suspicions; the Russians are planning a cyber attack against Britain by breaking into a submarine communications cable and shutting down every computer system in the country.
Although sceptical, Harry sends Lucas to Kachimov to learn which cable the submarine will attack so Malcolm can launch a counterattack.
[1] The plot of the episode was influenced by the resurgence of power in Russia following the end of the Cold War, which the producers felt in subtle ways is threatening the security of the West.
"[9] John Beresford of TV Scoop praised the entire episode, stating "the pace didn't let up from start to finish, the whole thing looked awesome, the acting was pitch perfect," and the dialogue "never drops into the realms of trite, or hyperbole, or parody.