Perkins goes 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. De Costa appears as a prison counsellor, acting as Perkins' contact with the outside world, but when Perkins opens up in a group therapy session, De Costa begins to suspect that he was abused as a child. By arresting a corrupt guard who was dealing drugs, Okaro hopes to force Mawdsley to play his hand, but in interview, the guard reveals that the prison's assistant governor is also corrupt. Meanwhile, Manson receives a call from a prisoner at the same nick who wants to pass on information about a planned robbery.
