When Jerry - an American NSA agent - joins a team at GCHQ tackling cyber crime, junior analyst Joseph is particularly excited. But Director of Cyber Security Christine is less keen with the shakeup and wants to know the real reason Jerry is over here. Meanwhile, Jerry looks to make his mark by leading a series of team building exercises.

When Joseph accidentally deletes the department personnel files the whole team are forced to redo their aptitude tests. Afraid of having to take a polygraph, Jerry manoeuvres to take over and run the test procedure himself. But when Jerry suspects that Christine may be hiding feelings for him he reluctantly agrees to taking the polygraph providing Joseph can secretly twist the results.

When Christine announces that the NHS servers have been hacked, the whole team are forced to work through the night. But finding the work too tedious, and directly going against Christine's orders, Jerry takes matters into his own hands and coerces the rest of the team into tracking down the culprit behind the attack his own way.

When GCHQ is hacked, Jerry suggests the person behind the cyber attack should join the team. But when Joseph becomes jealous of Jerry's cool new friend he looks to change his image and win back Jerry's affection. Meanwhile, shy cryptanalyst Mary gears up to seeing her ex-boyfriend for first time in 40 years who's been asked to deliver a seminar at GCHQ.

When Jerry admits that he's slept with department hacker Tuva, Joseph is devastated. But when Joseph finds out that Jerry is married he begins to question their whole friendship. Meanwhile, having failed her appraisal, Christine attempts to soften her style as Mary tries to single-handedly manage the department's workload.

When Jerry finds out his best friend and FBI agent Clint has been sleeping with his wife he accidentally shoots him. But when Clint shows up at GCHQ with an extradition order for Jerry, the truth about Jerry's NSA past comes out. Hoping it will grant Jerry British citizenship, Joseph and the team stage a wedding to protect Jerry and prevent him from leaving the UK.

The team are tasked with saving a nuclear power plant which is under cyber attack.

Jerry is hailed as the hero as disaster is averted at the nuclear power plant but threats still loom large. Meanwhile, the CySec mole is exposed.

It's a harassment and discrimination training day at CySec but all is not what is seems with the course leader, Hilary Bowen. Has Jerry met his match?

A rival GCHQ team find important information about the cyber weapon Eternal Blue. But when they refuse to share it, Jerry must think of a solution.

Jerry learns that a kidnapping group want him in an exchange for a hostage. Not keen on the idea, he comes up with a plan of his own.

After saving the day—and his own skin—Jerry is facing a bright future. Will he stay at GCHQ or go back to the US?

They’ve countered the Russians, they’ve foiled a plot to disrupt the NHS and they’ve prevented a nuclear disaster... But in this one-off special, the latest threat to our gang at the Government Communications Headquarters is personal. With the future of CySec at stake, the team have to go rogue to save their reputations and their jobs.