Claustrophobia. Resentment. Paranoia. Murder. 

H11 is an archive of state secrets so deadly it's kept four floors below ground in MI5's London HQ, completely isolated from the outside world and with no internet connection. It's staffed by the oddballs and rejects from the spy service.

When Lydia Twomey, a bright, young MI5 officer – no fan of the public school and Oxbridge-educated male elite who run the agency – is sent down to H11 to investigate the suspicious death of one of the staff, she finds a claustrophobic world ruled over by the charismatic but controlling Gerard Bell. She's quickly convinced that the 'accident' was murder and that one or more of the seven H11 staff were involved. After a second murder, Lydia becomes a target herself...

As the body count rises, it becomes clear that the stakes are much higher, and Lydia finds herself at the centre of a plot that could topple democracy in Britain.