Sonia Theobald is desperate.
Her husband Lou has been brutally murdered and the killer is threatening to do the same to her son Ashton if she doesn’t turn over a medieval broadsword the Theobalds uncovered under a hearthstone while remodeling their Arkham, Massachusetts mansion. Lou donated the sword to local Miskatonic University to study, but nobody has any idea how or why the sword got into the house, which was built by Lou’s namesake, a magistrate in the Salem witch trials. Also, soon after finding the sword, Lou started seeing his dead grandfather Henry walking around town and outside the mansion.
A friend gives Sonia a business card to The Haunted Bookshop and begs her to call them. “They don’t just sell books. Believe me. They helped me with a really bad problem, and they might be able to help you.”
So Sonia calls.
The shop’s manager, a rotund man named Stick, listens to Sonia, thanks her for calling and then contacts his boss, Feril Nightlinger, who is in New York City working as an effects and technical advisor on a new musical extravaganza, "Le Grand Méliès." After going through Stick’s information about Sonia’s problem Nightlinger decides to help, and he and his assistant, Mike Segretto, head to Arkham.
After retrieving (i.e., stealing) the sword from Miskatonic, Nightlinger has Mike return it to the Theobald mansion, where she is to guard Sonia and Ashton. Nightlinger recognizes that the sword was forged in eldritch fire, and, while researching local legends and regional newspapers at the campus library, pieces together information about a 17th-century coven that sacrificed to Susurrus, The Conqueror Worm, a primordial monster that survives by feeding off the corpses humanity buries in the ground. Susurrus sent send a piece of itself in the form of a Messenger to accept these sacrifices, which occurred in front of a megalithic structure on the outskirts of Arkham, until they were stopped by Judge Lewis Theobald, who used the eldritch broadsword -- the one type of weapon Susurrus is vulnerable to -- to destroy the Messenger.
Nightlinger drives to the megalithic structure, now part of a wooded cemetery, and spots Ashton being lured inside by a piece of Susurrus in the shape of a character named Lark from a fantasy book written by Sonia. When the pair reach a dead end Lark turns demonic and lunges at Ashton, but Nightlinger rescues the boy while Lark squeezes through a crack in a wall to escape.
Nightlinger is so angry at Mike for letting Ashton leave the mansion he refuses to listen to her explain that the boy was already gone when she got there and he sets out alone with the sword to dispatch Susurrus. Mike tails Nightlinger to the cemetery, waits while he digs up an empty grave that leads into one of Susurrus’ burrows and follows the trail to the ruins of an abhorrent cyclopean city. Nightlinger appears to intercept Mike but it is actually a piece of Susurrus that nearly strangles Mike before the real Nightlinger uses the sword to destroy the doppelganger. Mike is unconscious but alive so Nightlinger leaves her in a safe spot and enters the gigantic worm’s watery den. Like a knight battling a dragon Nightlinger cuts and slices Susurrus, but he nearly succumbs when the worm sends pieces of itself resembling Mike at him. Nightlinger recovers and destroys the doppelgangers, all of them screaming exactly like Mike, and the outraged Nightlinger drives the sword into Susurrus’ brain through an eye. Susurrus thrashes and dives under the water, knocking Nightlinger aside in its agony. Mike awakes to find Nightlinger waiting, ready to carry her to safety, but he is unable to guarantee that the ancient Susurrus has been destroyed forever.