John Carpenter’s In the Mouth of Madness has heady ideas it’s incapable of fully working out, but its tale – about John Trent (Sam Neill), an insurance investigator hired by a publishing firm to look into the disappearance of Stephen King-ish horror author Sutter Cane (Jürgen Prochnow) – is finely attuned to the disquieting realm of the irrational. Though Trent is constantly proclaiming that Cane’s popular books, which drive readers batshit crazy, aren’t “reality,” Carpenter’s story (penned by Michael De Luca) makes a rather blunt argument in favor of horror fiction’s power to influence via Cane’s latest novel “In the Mouth of Madness,” which has been inspired by demonic forces and seeks to drive the world into apocalyptic insanity. Trent discovers this while on a road trip to find Cane with publishing babe Linda Styles (Julie Carmen), a journey that leads him to Hobb’s Corner, a small New Hampshire town featured in one of Cane’s books and populated by rotting-flesh children and an unholy church presided over by the lunatic author himself. The line between the real and the unreal slowly melts away as Cane’s prose comes to life, but Madness’ portrait of art’s ability to manifest itself in the hearts and minds of its consumers is never quite lucid or well-paced enough to truly chill. That task, however, is ably taken up by Carpenter’s imagery of the impossible, from a nocturnal run-in with a boy (or is it an elderly man?) on a bicycle that’s defined by its ill-fitting elements, to a hotel lobby painting that mutates in dreadful ways. Even as his story devolves into a muddle, his acutely unsettling widescreen compositions thrillingly pinpoint the terror of the bizarrely incongruous.