Simon is a killer. Unfit to stand trial. He has been locked away for five years, watched by the sinister Dr Marlowe. Enter Karen, an enthusiastic but formidably intelligent psychologist. She will interview Simon - who wants to prove that he is cured. He seems sane. Seemingly incapable of ferocious acts of violence. He tells her of a terrifying force within him, which drives him to destroy those around him. He says this force has gone. Karen listens to his story. At first she trusts him, even believes him. But then her trust gives way to scepticism. His confession is replaced by deception. The interview unleashes demons from Simon's strange internal world - threatening Karen's previously stable sense of reality. She wants the truth. But she finds that truth can be the most terrifying thing of all.