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 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.