Detective Danny Arnold is part of a new team that has been formed to go underground in British prisons in order to discover how various crimes are being planned and by whom from within their walls. His target is George O'Brian, a particularly nasty character who has a history of extreme violence. Danny soon discovers that his target has links to a Russian gang, and is actively involved with the smuggling of illegal workers and drugs. He also discovers that O'Brian has ordered the killings of people from within prison. He practically runs the place. O'Brian is blackmailing the governor, who commits suicide as a result. After a coke binge O'Brian picks a fight with another inmate, but is seriously injured. He bribes an officer to plant a bag of cocaine in the man's cell. The two men have a confrontation and O'Brian is accused of raping new inmates. He denies the claim, but it later proves to be true after a medical examination on his latest victim.