Father Thomas Garrity is a Catholic priest who unexpectedly comes face to face with a horrific demon. Through fate or demonic manipulation, he winds up substituting for a prison chaplain who mysteriously committed suicide and was counseling a serial killer condemned on death row. Garrity meets with the convict, who at first is mute, and then becomes all too talkative, claiming to be a demon. A naive Father Garrity assumes he is dealing with a schizophrenic. Truly possessed by the demon Furfur, the inmate begins his attack on Father Garrity. As all Catholic priests, Garrity is uneducated and unskilled at confronting the supernatural, much less a demon. This demon is intent on destroying Garity's faith in God through any means possible. The one problem is that this "inmate" will be executed in a week, so time is of the essence. At first Garrity thinks that he must only survive until the execution, but then finds he will become hunted afterward by a demon that will transfer to a whole new body. Furfur, a documented fallen angel taken from demonology, initially attempts to overwhelm Garrity with his knowledge and history. Failing, he escalates his efforts to violence, and eventually murders. Father Garrity, as all common parish priests, is inexperienced, but is desperate to defeat this demon, even though his path is crammed with obstacles. He is surrounded by non-believers and a Catholic organization that has created an impossible bureaucracy which frowns on the very mention of exorcism. The demon Furfur intends to deliver the soul of this convict to hell and then transfer himself to another person and continue to torment the priest. Father Garrity finds that Furfur, in documented demonology, ranks high in the hierarchy of hell and has 150,000 demons at his command. Although in prison, Furfur can torture the priest through hurting those close to him simply by using his legions. Father Garrity reaches out to the Vatican and eventually enlists the help of the official head Exorcist. He is also aided by an unofficial American exorcist. Their realization that the limited time until the execution is nowhere near adequate for an exorcism, makes for an unsolvable dilemma.