Rose Elliot (Irene Miracle), a poet living alone in New York City, finds an ancient book titled The Three Mothers. The book, written by an architect named Varelli, tells of three evil sisters who rule the world with sorrow, tears, and darkness, and dwell inside separate homes that had been built for them by the architect. Mater Suspiriorum, the Mother of Sorrows, lives in Freiburg. Mater Lachymarum, the Mother of Tears, lives in Rome, and Mater Tenebrarum, the Mother of Darkness, lives in New York.
Rose suspects that she is living in one of the buildings and writes to her brother Mark (Leigh McCloskey), a music student in Rome, urging him to visit her. Using clues provided in the book as a guide, Rose searches the cellar of her building and discovers a hole in the floor which leads to a water-filled ballroom. She accidentally drops her keys and enters the water to find them. After she reclaims the keys, a putrid corpse suddenly rises from the depths, frightening her. Rose manages to get away, but is being watched.
In Rome, Mark attempts to read Rose's letter during class. He is distracted by the intense gaze of a beautiful student (Ania Pieroni), who leaves suddenly; Mark follows, leaving the letter behind. His friend Sara (Eleonora Giorgi) picks up the letter, and later reads it. Horrified by the letter's contents, she takes a taxi to a library and finds a copy of The Three Mothers. Sara is attacked by a monstrous figure who recognizes the book. She throws the book to the ground and escapes. Returning to her home, she phones Mark telling him to come and asks a neighbour, Carlo (Gabriele Lavia), to keep her company. The lights suddenly go out and both Sara and Carlo are stabbed to death by a gloved killer. Mark discovers the bodies and two torn fragments from Rose's letter. After the police arrive, he walks out of Sara's apartment and sees a taxi slowly driving by. In the back seat of the vehicle is the music student, staring at him intently again.
Mark telephones Rose but is unable to hear her clearly. He promises to visit just before the connection fails. Rose sees two shadowy figures preparing to enter her apartment. She leaves through a back door, but is followed. She is grabbed from behind by a clawed assailant and brutally guillotined across the neck with glass from a broken window.
Arriving in New York, Mark goes straight to Rose's building, where he meets Carol (Alida Valli), the concierge, and some of the residents, including a nurse (Veronica Lazar) who cares for elderly Professor Arnold (Feodor Chaliapin, Jr.), a wheelchair-bound mute. Mark encounters the sickly Countess Elise De Longvalle Adler (Daria Nicolodi), who tells him that Rose has disappeared. After the two find blood on the carpet outside Rose's room, Mark follows the stains. He becomes ill and falls unconscious. Elise sees a black-robed figure dragging Mark away, but the figure suddenly stops and gives chase to Elise. The figure eventually finds her swarmed with rabid cats in a room and stabs her to death. Mark staggers to the house's foyer where Carol and the nurse put him to bed.
Mark asks Kazanian (Sacha Pitoëff), the antique dealer who sold Rose The Three Mothers, about his sister's whereabouts. However, the man provides no information. That night, Kazanian drowns several cats in a Central Park pond and falls into the water. Hundreds of rats from a nearby drain crawl over him, gnawing his flesh. A hot dog vendor hears Kazanian's cries and rushes over, but proceeds to kill him with a knife. More strange deaths occur in the building, when Carol and Elise's butler, John (Leopoldo Mastelloni), plot to take advantage of the Countess' death by stealing her valuables. But a shocked Carol finds John's corpse in Elise's apartment, and drops a lit candle which starts a fire. Attempting to put out the flames, she becomes entangled in burning draperies and falls from a window to her death.
Mark uses a clue from Rose's letter to discover that beneath each floor is a secret crawl space. He follows hidden passages to a suite of rooms where he finds Professor Arnold who reveals, via a mechanical voice generator, that he is in fact Varelli. He tries to kill Mark with a hypodermic injection. During the struggle, Varelli's neck is caught in his vocal apparatus, choking him. Mark frees him, only to be told by the dying man, "Even now you are being watched." Mark follows a shadowy figure to a lavishly furnished chamber, where he finds Varelli's nurse. Laughing maniacally, she reveals to him that she is Mater Tenebrarum. She suddenly transforms into Death Personified. However, the fire that has consumed much of the building enables Mark to escape from the witch's den. Debris crashes down on the fiend, destroying her.
In 1977, Suspiria had been an unexpectedly big box office hit for 20th Century Fox, released in the U.S. under their "International Classics" banner. Capitalizing on the commercial success of the film, Argento and Daria Nicolodi, who had co-written the screenplay, announced that Suspiria was only the first of a proposed trilogy which they referred to as "The Three Mothers" trilogy. The basic concept of all three films is derived from Thomas de Quincey's Suspiria de Profundis, a sequel to his Confessions of an English Opium-Eater. A prose poem of the book entitled "Levana and Our Ladies of Sorrow", details how, just as there are three Fates and three Graces, there are also three Sorrows: Mater Lachrymarum (The Lady of Tears), Mater Suspiriorum (The Lady of Sighs) and Mater Tenebrarum (The Lady of Darkness). As the title suggests, Suspiria focused on Mater Suspiriorum; the evil sister featured in Inferno is Mater Tenebrarum. The concluding chapter of Argento's trilogy, The Mother of Tears (2007), is about Mater Lachrymarum.
When Argento proposed Inferno as his follow-up to Suspiria, 20th Century Fox agreed to co-finance the production. The film was budgeted at USD $3,000,000, and producer Claudio Argento secured additional co-production money from Italian and German consortia.
Nicolodi devised the original story concept but received no on-screen credit for her work on the screenplay. Nicolodi explained that she did not seek credit because "having fought so hard to see my humble but excellent work in Suspiria recognized (up until a few days before the première I didn't know if I would see my name in the film credits), I didn't want to live through that again, so I said, 'Do as you please, in any case, the story will talk for me because I wrote it.'" Working from Nicolodi's original story notes, Argento wrote the screenplay while staying in a New York hotel room with a view of Central Park.
The filming of Inferno took place mainly on interior studio sets in Rome, but a short amount of time was also set aside for location shooting in New York, including Central Park. Sacha Pitoëff's death scene was filmed on location in Central Park during the summer of 1979. William Lustig, who was credited as the film's Production Coordinator, recalled,
They filmed the actor carrying a bag that contained some kind of moving mechanism, to make it look like it was full of cats. He walked into the lake, pushed the bag underwater, and fell in. At that point, some phony mechanical rats were attached to him for closeups. When the guy at the hamburger stand runs over the lake... that guy was actually running on a plexiglass bridge under the water; it made it look like he was actually running across the surface of the lake. All of the stuff with the live rats was shot back in Europe.
During the film's production, Argento became stricken with a severe case of hepatitis---and had to direct some sequences while lying on his back. At one point, the illness became so painful that he was bedridden for a few days; filming was then restricted to second unit work, some of which was done by Mario Bava. Argento has called Inferno one of his least favorite of all his films...as his memories of the movie are tainted by his recollection of the painful illness he suffered.
Argento invited his mentor, Mario Bava, to provide some of the optical effects, matte paintings and trick shots for the film. Some of the cityscape views seen in Inferno were actually tabletop skyscrapers built by Bava out of milk cartons covered with photographs. The apartment building that Rose lived in was in fact only a partial set built in the studio—it was a few floors high and had to be visually augmented with a small sculpture constructed by Bava. This sculpture was set aflame toward the end of production and served as the burning building seen in the climax.
Bava also provided some second unit direction for the production. Maitland McDonagh has suggested that Bava had his hand in the celebrated watery ballroom scene, but that sequence was shot in a water tank by Gianlorenzo Battaglia, without any optical effects work at all. Bava's son, Lamberto Bava, was the film's assistant director.
The film's fiery final sequence was shot without a stunt performer filling in for Leigh McCloskey. After the production's principal photography had been completed, the film's producer, Claudio Argento, asked if McCloskey would be willing to perform the stuntwork himself, as the stuntman hired for the job had broken his leg. The producer assured the actor: "It'll be absolutely safe". The actor agreed, and when he walked onto the set the following day he observed "three rows of flexiglass in front of everything and everyone is wearing hard hats. I'm the only guy standing on the other side of this! ...Needless to say, I did it all on instinct... I still feel that blast of the door blowing by me. When they tell you in words, its one thing, but when you feel that glass go flying past you with a sound like a Harrier jet, you never forget it!"
Dario Argento chose progressive rocker Keith Emerson to compose Inferno's soundtrack because he "wanted a different sort of score [from that by Italian prog group Goblin on Suspiria], a more delicate one".
Argento prominently featured a selection from Giuseppe Verdi's Nabucco throughout Inferno, the Chorus of the Hebrew Slaves ("Va, pensiero, sull'ali dorate"), an operatic chestnut, from scene two of the opera's third act. In two instances, a recording of the Sinphonic [sic] Orchestra and Chorus of Rome was used. Argento also tasked Emerson with including the piece in his soundtrack. He re-orchestrated "Va, pensiero..." in five-four time to mimic a "fast and bumpy" taxi ride through Rome. When Argento reviewed Emerson's progress he did not initially recognize the remix, but was later pleased to discover it was used for Sara's taxi ride.
A soundtrack album was originally released as an LP by the Cinevox label in 1981. In 2000, Cinevox released an expanded version of the album on CD.
Emerson's music met with a mixed response from critics, some of whom compared it unfavorably to Goblin's score for Suspiria. Time Out's Scott Meek noted that "Argento's own over-the-top score [for Suspiria] has been replaced by religioso thunderings from the keyboards of Keith Emerson". A review of the 2000 Cinevox CD by AllMusic notes, "The keyboard selections are rather unremarkable, except for the finale, "Cigarettes, Ice, Etc.", on which Emerson uses his full keyboard arsenal to excellent effect. Unfortunately, the choral segments sound rather pretentious and dated."
For reasons never specified, Fox did not commit to a wide theatrical release of Inferno in the United States. In an interview with Maitland McDonagh, Argento speculated that Fox's decision was made due to an abrupt change in management at the studio that left Inferno and several dozen other films in limbo as a result of them having been greenlighted by the previous management. The movie sat on the shelf for five years and was released straight to VHS in 1985 via the studio's Key Video subsidiary. The following year, it had a belated theatrical release by Fox, playing for a one-week engagement in a New York City movie theatre. Worldwide, the film only had a very abbreviated and minimal theatrical release. As noted by Argento, "I think anybody outside of Italy was lucky to see Inferno." Consequently, Inferno was not a commercial success.
Initial critical response was fairly muted. Several reviewers expressed disappointment, comparing the film unfavorably to the much more bombastic Suspiria. Scott Meek in Time Out said that of the two movies, Inferno was "a much more conventional and unexciting piece of work [...] the meandering narrative confusions are amplified by weak performances." In a review that was later reprinted in McDonagh's critically acclaimed Broken Mirrors/Broken Minds: The Dark Dreams of Dario Argento (1994), Variety said Inferno was a "lavish, no-holds-barred witch story whose lack of both logic and technical skill are submerged in the sheer energy of the telling", then complained that the film "fails mainly because it lacks restraint in setting up the terrifying moment, using close-ups and fancy camera angles gratuitously and with no relevance to the story." Reviewing the film during its brief theatrical release in 1986, Nina Darnton of The New York Times noted, "The movie's distinguishing feature is not the number or variety of horrible murders, but the length of time it takes for the victims to die. This is a technique that may have been borrowed from Italian opera, but without the music, it loses some of its panache. The film [...] is shot in vivid colors, at some striking angles, and the background music is Verdi rather than heavy metal. But the script and acting are largely routine." However, Cinefantastique described the film as "the stuff of all our worst dreams and nightmares and a tour de force from Italian director Dario Argento[...] Inferno brings his personal redefinition of the genre close to perfection."
Inferno continues to have a mixed critical reputation. The film has a 64% favorable rating on the "Tomatometer" at movie review aggregator website Rotten Tomatoes, out of twelve surveyed internet film reviewers. Leonard Maltin's Movie Guide gave the film two and a half stars and opined it was a "surreal, hypnotic shocker...short on sense, but long on style." But several critics have praised the film. Upon its initial release on videotape, Tim Lucas in The Video Watchdog Book said, "The movie is terrific, much more exciting than most contemporary horror video releases..." Kim Newman, in The Penguin Encyclopedia of Horror and the Supernatural, noted that Inferno was "a dazzling series of set pieces designed to give the impression that the real world is terrifying, beautiful, erotic and dangerous[...] Inferno is a masterpiece of absolute film, and perhaps the most underrated horror movie of the 1980s." In 2013, Time Out compiled a list of the 100 greatest horror films ever made based on the top ten lists of over one hundred film directors, screenwriters, and critics, and Inferno was listed as #92; in the resulting critical commentary for the film, Nigel Floyd wrote, "Horror cinema at its most baroque: a simple libretto is embroidered with elaborate, flowing camera movements, abstract blocks of colour, unsettling sound effects and soundtrack composer Keith Emerson’s thunderous rock variations on Verdi...Argento’s best work is far behind him, but this alone justifies his cult reputation."