When John Peter Barrie escapes from a maximum security mental institution, the police assign all available resources to finding him. Barrie is a dangerous sex offender known as the Devil's Disciple. Morse is assisted by Barrie's psychiatrist, Dr. Esther Martin. Barrie contacts Morse and they begin a deadly game of cat and mouse. When Morse refuses to play the game Barrie's way, the madman kidnaps a woman and demands that Dr. Martin be held for a future exchange. As the investigation proceeds, Morse believes Barrie has accomplices in the area but after a local cultist is killed, he realizes he has been looking in wrong direction all along.