Henry Plocett, a rude and over-bearing patient at St. Matilda's Psychiatric Hospital in London is found dead at the foot of a staircase. It is thought that his death is an accident until the hospital administrator, Peter Cathacart examines the scene. Pointing out to the medical director that, as the entrances to the staircase are code controlled, a member of staff must be involved, the only recourse is to call the police. Superintendent Malden and sergeant Stanton must now unravel a complex and sinister chain of events involving forgery, rape and, ultimately, murder.