Someone is hacking into M15's central computer. Files are disappearing and agents are being lost in the field.The number one candidate is a secondary school teacher, Gordon Blaney (Chris Fairbank - Auf Wiedersehen, Pet, Crocodile Shoes), who is face-matched near an offending computer. M15's files link him to a violent socialist group, the SFM, and terrifying assaults and protests.The SFM is thought to have disbanded some time ago; has it regrouped? Posing as a supply teacher, Zoe is placed in the school in order to get near Blaney. Danny, meanwhile, tries to infiltrate the SFM. Zoe enjoys normal life in the community, getting to know the children etc, but she has doubts about Blaney's politics and whether he is the hacker. Meanwhile, the central computer is being hacked into more and more, and it's not clear whether two agents in Pakistan are alive or dead. Just as the team think they are near to breaking Blaney, it becomes clear they have been looking at the wrong man. Could it be someone who has been under their noses, but who really doesn't fit the mould? Guest stars in this episode also include William Gaminara (Casualty,Attachments) and John Salthouse (The Bill).