Danny is under huge pressure dealing with the fallout from the Russian cyber-attack when a bold new threat emerges. Saara discovers that there's a mole in GCHQ. And Vadim is missing and in danger...

Vadim realises that he must try to escape captivity in Shenzhen. Back at GCHQ, Danny and Roz clash over China. And has Charlie unearthed the mole?

When Barbara's charming ex-boyfriend, Nicky Chan, re-appears in her life, she must make an impossible decision.

While Danny becomes increasingly concerned about Barbara's erratic behaviour, malware causes havoc across British government departments, forcing Danny and Roz to work together

Barbara comes under Charlie's scrutiny as tragedy strikes GCHQ. Danny, Saara and Roz attempt to help Vadim escape. And Danny makes an earth-shattering discovery.

Danny puts everything on the line in order to right some wrongs. Meanwhile, Roz and the team must race against the clock to find out what the malware is searching for.