Vadim returns to Russia where his allegiances are tested, leading him to a risky stunt.

As the true extent of Russia's interference with the UK becomes clear, Saara and her former classmate join forces. Can they uncover the hidden code and prevent a war?

Saara befriends John, a veteran GCHQ staffer, and seeks help from Gabriel in decrypting a message in the virus. Danny signs off on Saara attending a cyber conference with US agent Kathy, aware that Russian intelligence will also be there.

With Saara in a difficult position, a revelation forces her hand. Marina moves to London.

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.

Signs of irregularity in the UK general election result prompt political unrest, and the nation's relationship with its allies is threatened. Saara makes a devastating discovery.