In the Mouth of Madness is a 1994 American supernatural horror film directed and scored by John Carpenter and written by Michael De Luca. It stars Sam Neill, Julie Carmen, Jürgen Prochnow, David Warner and Charlton Heston. Neill stars as John Trent, an insurance investigator who visits a small town while looking into the disappearance of a successful author of horror novels, and begins to question his sanity as the lines between reality and fiction seem to blur. Informally, the film is the third installment in what Carpenter refers to as his "Apocalypse Trilogy", preceded by The Thing (1982) and Prince of Darkness (1987).^[2]

In the Mouth of Madness pays tribute to the works of author H. P. Lovecraft in its exploration of insanity, and its title is derived from the Lovecraft novella At the Mountains of Madness. Distributed by New Line Cinema, In the Mouth of Madness received mixed reviews upon release.

In the midst of an unspecified disaster, Dr. Wrenn visits John Trent, a patient in a psychiatric hospital, and Trent recounts his story:

Trent, a freelance insurance investigator, has lunch with the owner of an insurance company, who asks Trent to work with his largest client investigating a claim by New York-based Arcane Publishing. During their conversation, Trent is attacked by a man with mutated eyes wielding an axe who, after asking him if he reads popular horror novelist Sutter Cane, is shot dead by police. The man was Cane's agent, who went insane and killed his family after reading one of Cane's books.

Trent meets with Arcane director Jackson Harglow, who tasks him with investigating the disappearance of Cane and recovering the manuscript for his final novel. He assigns Cane's editor, Linda Styles, to accompany him. Linda explains that Cane's stories have been known to cause disorientation, memory loss and paranoia in "less stable readers". Trent is skeptical, convinced the disappearance is a publicity stunt. Trent notices red lines on the covers of Cane's books, which, when aligned properly, form the outline of New Hampshire and mark a location alluded to be Hobb's End, the fictional setting for many of Cane's works.

Linda experiences bizarre phenomena during the late-night drive, and they inexplicably arrive at Hobb's End in daylight. Trent and Linda search the small town, encountering people and landmarks described as fictional in Cane's novels. Trent believes it all to be staged, but Linda disagrees. She admits to Trent that Arcane's claim was a stunt to promote Cane's book, but the time distortion and exact replica of Hobb's End were not part of the plan.

Linda enters a church to confront Cane, who exposes her to his final novel, In the Mouth of Madness, which drives her insane; she begins embracing and kissing a mutated Cane. A man who had previously attempted to stop Cane's insidious agenda approaches Trent in a bar and warns him to leave, then commits suicide. Outside, a mob of monstrous-looking townspeople descend upon Trent. Trent drives away, but is repeatedly teleported back to the center of town. After crashing his car, Trent awakens inside the church with Linda, where Cane explains that the public's belief in his stories freed an ancient race of monstrous beings called "The Old Ones" which will reclaim the Earth. Cane reveals that Trent is merely one of his characters, who must follow Cane's plot and return the manuscript of In the Mouth of Madness to Arcane, furthering the end of humanity.

After giving Trent the manuscript, Cane tears himself and a wooden door behind him open as if they were paper, creating a portal to the dimension of Cane's monstrous masters. Trent sees a long tunnel that Cane said would take him back to his world, and urges Linda to come with him. She tells him she can't, because she has already read the entire book. Trent races down the hall, with Cane's monsters close on his heels. He suddenly finds himself lying on a country road, apparently back in reality. During his return to New York, Trent destroys the manuscript. Back at Arcane, Trent relates his experience to Harglow. Harglow claims ignorance of Linda; Trent was sent alone to find Cane, and the manuscript was delivered months earlier. In the Mouth of Madness has been on sale for weeks, with a film adaptation in post production. Trent then encounters a reader of the newly released novel, who is bleeding from his altered eyes; Trent murders him with an axe. Trent is then arrested for murder and sent to the asylum.

After Trent finishes telling his story, Dr. Wrenn judges it a meaningless hallucination. Trent wakes the following day to find the asylum abandoned, and he departs as a radio announces that the world has been overrun with monstrous creatures, including mutating humans, and that outbreaks of suicide and mass murder are commonplace. Trent then goes to see the In the Mouth of Madness film where he discovers that he is the main character. As he watches his previous actions play out on screen - including a scene where he insisted to Linda "This is reality!" - Trent begins laughing and crying hysterically, finally realizing that he is just a work of fiction.

In addition, Frances Bay plays Mrs. Pickman, while Wilhelm von Homburg appears as Simon. Hayden Christensen makes his film debut, briefly seen as a paper boy.

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Michael De Luca wrote the script in the late 1980s and one of the first directors he offered it to was John Carpenter,^[3] who initially passed on the project. New Line Cinema later announced production in 1989 with director Tony Randel to direct.^[3] Later, Mary Lambert was also attached to direct.^[3] Finally, Carpenter signed on as director in December, 1992, and filming took place from August to October, 1993.^[3] The film had a budget of approximately $8 million.^[4]: 21

The town scenes in Hobb's End were filmed on Main Street Unionville, and the exterior of the Black Church is actually the Cathedral of the Transfiguration. Both are located in Markham, Ontario.^[5] The rest of the film was shot on location in Toronto, Ontario due to its unique mix of "New York skyscrapers and New England remoteness" according to Carpenter.^[6]

The visual effects for the film were done by Industrial Light & Magic, and the practical effects (including creature prosthetics and animatronics) were done by the KNB EFX Group. It took seven weeks for KNB to create all the practical effects for the film, the biggest of which was an "eighteen-foot Wall of Monsters" that was mounted on rollers and operated by a crew of twenty-five people.^[7][8] "We had under seven million [dollars] to spend on the picture", Carpenter said in 2001. "It was tough. The monsters had to be outrageous Lovecraftian demons, but in H.P. Lovecraft's novels, they are always so horrible that they are beyond description, that they are too terrifying to see. So how do you visualize something like that? Well, very, very quickly. They also had to be slightly cartoonish since Sam Neill spends most of the movie ridiculing horror only to discover it's all too real."^[9]

Following the early VHS releases, a Blu-ray version of the film by New Line Cinema was released in 2013.^[25] In 2016, the film was re-released on DVD by Warner Archive Collection.^[26] In 2018, Shout! Factory re-released the film under its Scream Factory sub-label as a Collector's Edition Blu-ray.^[27]