A young intern's first day at GCHQ coincides with a major cyberattack on the UK. To win her superiors' trust, she breaks ranks to analyse the malware code without permission and uncovers a greater threat missed by others.

Saara is put to work combing through the text in the virus, looking for passwords. It's a monotonous job, but after many hours she finds a suspicious line of encrypted text within the code. Her colleagues think it's unimportant, but Saara isn't convinced. She also gets more familiar with the players at GCHQ and makes some unlikely friends. With their help, Saara unravels a dubious invitation to rendezvous - but is she closer to the truth or just willingly entering a carefully baited trap?

Saara's classmate Vadim returns to St Petersburg. He reluctantly takes a covert job and soon becomes the victim of his own success as his allegiances are put to the test, leading him to a risky stunt.

With Danny unconvinced by Saara's exchange with a whistleblower, she's in a difficult position, but a revelation from Kathy forces Saara's hand. Meanwhile, Marina takes up a new opportunity in London.

The UK general election result prompts unrest, while Saara makes a devastating discovery.

As the truth becomes clear, Saara and her classmate join forces to prevent a war.