In the 70's there were many films about Satan ("The Exorcist, "The Omen") and many films about insane asylums ("One Flew over the Cuckoo's Nest", "The Ninth Configuration"). It's no surprise then that someone would combine them. But it's a testament to the prescient genius of William Girdler--the man, the myth, the legend--that he did so in 1971! Of course, this film is completely inane, but so were all Girdler films, even the ones with big budgets and big stars.
A woman has a nervous breakdown and wakes up in an asylum. She finds it strange (although not nearly as much as she ought to) that all of her fellow patients are dressed in white-hooded robes. Meanwhile, her boyfriend, who looks like "Meathead" from all in the family and is garbed in some of the worst 70's fashions ever witnessed, is trying to rescue her. After being turned away the first time, he comes back with a police detective and discovers that the asylum is abandoned and the doctor he spoke to, Dr. Spector, has been dead 20 years. He's very alarmed, however, when he finds out that the supposedly dead Dr. Spector "was picked up several times for worshiping Satan" (what's the police code on that--a "666"?). Several minor characters are killed for no other reason than to show off some ridiculous-looking special effects (paper bugs, rubber snakes, etc.). As in "Three on a Meathook" there's also a long romantic flashback interlude set to music (because horror fans, of course, are suckers for romantic interludes). Finally though, a guy in a VERY unconvincing Satan mask shows up and we learn that it's not good to offer Satan a non-virgin in a virgin sacrifice (he apparently hates getting beaten to the punch by a guy who looks like "Meathead").
Obviously, this movie is pretty bad, but it almost sneaks into so-bad-it's-good territory. I'd definitely recommend it for fans of satanism, insane asylums, and William Girdler.