When Jerry - an American NSA agent - joins a team at GCHQ tackling cybercrime, 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.

Joseph accidentally deletes the department personnel files, forcing the team to redo their aptitude tests. Afraid of having to take a polygraph, Jerry attempts to run the test procedure himself - 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.

GCHQ is hacked. Jerry suggests the person behind the cyberattack 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.

Jerry makes a confession that devastates Joseph and leads to him questioning their whole friendship. Having failed her appraisal, Christine tries a new approach.

A shock turn of events outs Jerry's past. As his job at GCHQ comes under threat, Joseph hatches an elaborate plan to keep him in the UK.