“Suspiria” and “Inferno,” the first two installments of Dario Argento’s “Mothers” trilogy, appeared in 1977 and 1980, and while both tales of witchcraft were short on logic they were certainly long on the hypnotically extravagant, operatic style that the Italian director had developed over his earlier string of gialli; they were so beautifully nightmarish their implausibility barely mattered. It’s taken more than a quarter century for him to get around to finishing off the triptych with “Mother of Tears: The Third Mother,” and unfortunately the passage of time has not been kind. In this case the ludicrousness not only reaches astronomical proportions, but Argento’s command of appearances fails him as well. The picture does not amaze visually, with even the elaborate death sequences coming across as laughably ugly rather than ghoulishly dreamlike.

The plot involves the last of the three ancient witches who once ruled the world, the first two—the mother of sighs and the mother of darkness—having been disposed of in the previous installments, set in Freiburg and New York City, respectively (where those witches had taken up residence). The reign of the mother of tears begins when an ancient urn is uncovered, along with a nineteenth-century coffin, in unconsecrated ground adjacent to an Italian cemetery. The local monsignor sends it to a museum in Rome for inspection, and when intern Sarah (Asia Argento) and her colleague Giselle (Coralina Cataldi Tassoni) open it, they discover three little statues, a shroud and a knife. When Sarah goes off to collect books that Giselle has asked for—absurdly enough, “Aramaic and Mycenaean dictionaries”!—demons appear and rip Giselle to shreds (terrible bowels-falling-out scene), watched closely by a screeching monkey that apparently has an evil pedigree. In fact, the simian attempts to catch Sarah as she escapes the museum.

But she does get away, only to discover not only that the opening of the urn has unleashed an epidemic of violence—murders, suicides, rapes—on the city (initiating a rash of requests for exorcisms!) but that it’s also started an influx of witches from all over the world to Rome, which they plan to take over under the third mother’s reign. Moreover, Sarah discovers that she herself is the daughter of the good witch who destroyed the mother of darkness and (as her dead mom telepathically informs her) she must learn to use her inherited powers to defeat the third and final sister/mother.

What follows is a chaotic series of set-pieces in which various characters are eviscerated or bludgeoned to death, Sarah is stalked by would-be assassins, and life apparently goes on pretty normally despite the killings that are supposedly happening everywhere. Meanwhile the witches keep flooding into Rome—apparently all Goth mean girls who do nothing but flounce around the streets shrieking and giggling and making passersby uncomfortable. Eventually our heroine winds up in the witch’s underground lair, where her followers dismember captives using very poor special effects and engage in orgiastic activities about seven steps below the ones Kubrick depicted in “Eyes Wide Shut.” In a distinct letdown of an ending, she defeats the purportedly all-powerful witch with what amounts to a flick of the wrist.

The broad outline of “Mother of Tears” is ridiculous, but it’s the wacky details—monkeys, Goth girls, gruesomely hilarious death scenes (Sarah disposes of one witch by bashing her head repeatedly with a sliding lavatory door)—that take the picture into complete lunacy. By the howlingly funny ending it’s become insta-camp that practically begs for midnight screenings and talk-back audiences.

The narrative nonsense is bad enough, but it’s exacerbated by the gabble on the soundtrack. The movie’s been shot in English, but most of the actors are either badly dubbed, or speak in heavy accents with inflections that suggest they’ve learned their lines phonetically. Their performances tend to be wildly over-the-top, with Argento, Dario’s daughter, leading the way as the put-upon Sarah. But the gaggle of witches aimlessly wandering Rome’s streets take the cake. As for Moran Atias, the Mother of Tears herself, she’s actually a minor figure under the final reel, when she demonstrates both an inability to recite dialogue in even a remotely convincing fashion and a willingness to disrobe (a trait that seems common among her female followers—gratuitous nudity is a motif here). Adding to the overwrought atmosphere is a pounding chorus-and-orchestra score by Claudio Simonetti that apes Jerry Goldsmith’s “Omen” music, but at a higher decibel level.

Argento devotees who have been awaiting the capstone of his trilogy for years, even decades, are bound to be disappointed by “Mother of Tears.” The visual mastery that not only excused but actually elevated the illogic of the first two installments of the trilogy is absent here. This is a risible clunker, and those who have long admired the director can only hope that it was intended as a joke rather than a culmination of his career.