DC Terry Perkins is undercover in prison, where he investigates convicted paedophile Charles Mawdsley who is believed to be running a paedophile ring with the help of one of the prison guards. DS Ramani de Costa appears as a prison counsellor, acting as Terry's contact with the outside world, but when Terry opens up in a group therapy session, Ramani begins to suspect that he was abused as a child. By arresting a corrupt guard who was dealing drugs, Supt Okaro hopes to force Mawdsley to play his hand, but in interview, the guard reveals that the prison's assistant governor is also corrupt. Okaro and de Costa race to the prison just in time to stop Terry from being killed by Mawdsley. Meanwhile DI Neil Manson gets a call from a prisoner at the same nick who wants to pass on information about a huge planned diamond robbery - the grass is none other than Don Beech, Manson's former sergeant at Barton Street and former Sun Hill DS.
