Twinkle, Twinkle, "Killer" Kane! 
Written by William Peter Blatty
The Ninth Configuration 
Written and Directed by William Peter Blatty
The Ninth Configuration is a film destined for the Cult Classics section of the video store. It's about Vietnam and veteran readjustment. It's about sanity in an insane world. It's a wacky/serious psychiatric hospital picture in the vein of King of Hearts or One Flew Over the Cuckoo's Nest. It's a Gothic "dark, scary castle" picture. It's a Catholic discourse on God's silence in response to the evil that plagues creation. And it's a sequel to The Exorcist about a character who got five minutes of screentime in that film...and was upstaged by Linda Blair pissing on the carpet.
That character is the astronaut to whom Blair's Pazuzu-possessed Regan remarks, "You're going to die up there." His name is Captain Cutshaw and on the launching pad of his moonshot he had a bit of a freak out. Now he's in an imported German castle in the Pacific Northwest that the Marines have converted into an experimental asylum for vets who lost it in 'Nam. Are Cutshaw et al. really beyond the pale, or are they faking it to stay out of harm's way? Marine psychiatrist Colonel Kane is sent to assess the men and evaluate if they can return to duty or not. But, not surprisingly, Kane himself has terrible secrets that are roaring to escape.
Sharp-eyed readers will note that Kane was written and published before The Exorcist, and that the astronaut in that book/film was unnamed. Correct and correct again. After the phenomenal success of The Exorcist, author Blatty used his newfound Hollywood clout to write and direct the film version of Kane. He got a better (and spoiler-free) title, updated the story, dispensed with some unnecessary (and relatively flat) buffoonery involving a paranoid senator and the general hounding him for funding, and shifted the locale a few hundred miles north. The uneven tonal shifts of the novel are finessed and equalized; the decision has been made to take things a bit more seriously from the get go, and we're clued in to that fact by the film's moody atmosphere and music. There are still some smart bits of comedy (the dogs learning Shakespeare bits still amuse me), but an outer darkness rings and reins in the absurdity. Oh, and there's a kick ass bar fight and a minor miraculous denouement.
The Ninth Configuration is the second in Blatty's "Trilogy of Faith," being followed by The Exorcist III. (Sorry, John Boorman's literally abysmal The Exorcist II: The Heretic has no place within the Blatty canon.) Each film deals with the problem of retaining faith when contested by evil. The Exorcist portrays spiritual evil, The Ninth Configuration portrays human evil, and Exorcist III combines the two in the form of a Satanic serial killer. In each, a troubled Catholic uses the wickedness around him to reaffirm his faith in the one true church. There's usually a martyrdom in there somewhere, too.
Admittedly, it can all get a bit heavy handed. I had to grit my teeth when Cutshaw ecstatically muttered, "He gave his life for us." And as viewers we're never really in doubt that the path of righteousness will be proven true and just. But the acting's good, the dialogue (when not devoted to sermonizing) is sharp, and the sum of the parts yields a truly unhinged spiritual redemption film for which there is no real comparison.
If you're up for some gonzo Catholicism and you need a break from Mel Gibson, give The Ninth Configuration a try. After all, where else are you going to see Christ crucified on the moon?