In early 1980's Italy, a serial killer nicknamed Bauta (after their distinctive costume: a dark blue cape, fedora, and a black bauta mask) is arrested; the culprit is revealed to be a young Army private named Armstrong. He is tried and found guilty at the local city courthouse. As he is being escorted out by officers Jan Tadeo and Sarah Fabrizzi, Armstrong is shot twice by a mysterious assailant across the street. Fabrizzi gives chase as Jan attempts to stop the bleeding. Armstrong whispers the phrases "polynikus seori culcana" and "stay away from Seveka" before dying. Fabrizzi returns empty-handed as the police secure the scene.
Five years later, transit worker Leon Pedicone - Armstrong's shooter - is finishing up his shift cleaning up an empty subway train. Pedicone seemingly spots a shadowy figure and decides to leave early. As he climbs up the station stairs, he is attacked by Bauta and falls backwards to his death. He leaves a dying message: Seveka L4. Pedicone's murder is being investigated by Tadeo, who has quit the force and become a private investigator, and now-detective Fabrizzi as a continuation of the serial killings. Connecting the dying messages of Armstrong and Pedicone, Tadeo locates Seveka; a small coastal town. He drives to Seveka and rents a room at the local inn, owned by resident Antony Stevan. Tadeo unpacks and sees the figure of Armstrong standing in the street. He rushes out but he figure disappears before he gets there. As he re-enters the inn, Tadeo notices a photograph on the wall depicting Armstrong, Pedicone, and Aust Hanalei - another one of Armstrong's original victims. Tadeo calls Fabrizzi to let her know about the leads before going to bed.
When Tadeo awakens the next morning, he asks Stevan about the picture. Stevan explains he doesn't know much about them except for Hanalei's wife, Rebecca, who rents a lodge from him. Fabrizzi shows up minutes later and they talk about the new leads. She takes him back to the city to see new evidence and to discuss the resurgence of the killer and their motives. They stop at a food stand, and Tadeo spots Armstrong standing by Fabrizzi's car; it blows up when she tries to re-park her car. Tadeo nervously takes a cab back to Seveka and continues with his investigation. He goes to the library to acquire a book on ancient Gothic - the language the phrase Armstrong muttered was in - and also gets Rebecca's address as well. Tadeo heads over to Rebecca's place and attempts to question Rebecca, but she flatly denies knowing anything about the skeleton cliff or Lodge Four, confusing Tadeo. He returns to the inn to retire for the night. Rebecca is violently murdered when her Achilles tendons are cut and she is set on fire.
Seveka's law enforcement, Sheriff George Banson and Deputy Alberto Gastaldi investigate Rebecca's murder. They question Tadeo on his knowledge of the killings. Gastaldi strongly suggests that Tadeo should leave Seveka before he gets in too deep. Banson and Gastaldi also show him the skeleton cliff: a rocky edge of shoreline with approximately a dozen rotting corpses littered on it. Back at the inn, Stevan tells Tadeo about the lodge he rented out to the Hanaleis, Lodge Four, and the old lighthouse it connects to. Tadeo returns the book back to the library and learns more about the diety Usholoda and their cult worshippers, the Unhultians, and their relation to Lodge Four. Tadeo spots Armstrong at the library and hurriedly leaves. Librarian Courent closes up the library after him, but is ambushed by Bauta; they strangle, hang, and slash at her until she dies.
Tadeo is put into temporary confinement by Banson under suspicion of being the murderer; Tadeo's keys are taken from him. Restless and in an investigatory mood, Tadeo sneaks out and goes to Lodge Four. Gastaldi notices and follows Tadeo without notifying anyone. When Tadeo arrives, he finds that Lodge Four is filled with blood and viscera. He also discovers a sign-in sheet, and reading off the names, he realizes that everyone that has been killed so far has been part of Lodge Four. The sheet has been torn off at the bottom; he also notices a picture missing from the lodge. As Tadeo leaves to explore the lighthouse, Gastaldi arrives at the Lodge; Bauta severs Gastaldi's arm. The lighthouse lobby is filled with paintings, which Tadeo momentarily looks at before he hears Gastaldi's scream. Tadeo rushes to the Lodge and finds Gastaldi nearly dismembered. Nervous, he steals Gastaldi's revolver before sneaking back to the inn. Sheriff Banson - still thinking that Tadeo was in the inn the entire time - apologizes for suspecting he was the murderer. The two talk about the case so far, including the lighthouse and Fabrizzi's death. Banson gives Tadeo his number and tells him to stay safe: Tadeo wishes Banson the same. That evening, Tadeo explores Lodge Four thoroughly. He finds the missing photo from earlier and recognizes someone in it; he immediately leaves and rushes back to the inn. However, Bauta appears and starts chasing Tadeo through the empty streets, nearly catching him before he ducks back into the Inn. Stevan's dead body lays splayed out on the front desk.
A phone rings at the desk, which Tadeo answers. The caller is Banson. Tadeo explains what is happening to him, and Banson tells Tadeo he was something important to show to him - and to meet him at the lighthouse as soon as possible. Tadeo goes to his room to retrieve the revolver, but he hears Bauta entering the inn. Not taking a chance, he jumps out of the window, spraining his ankle, and limps his way to the lighthouse. He arrives at the lighthouse, but notices something is amiss. He realizes that there is a door missing, and a painting is slightly off. Tadeo pulls on the painting and reveals a secret passage. It leads to a seaside cavern with an altar; searching around, he finds the full sign-in sheet from the Lodge. He is about to explore more when he hears a scream from the top of the lighthouse. Tadeo hurriedly climbs the stairs, seeing Banson being murdered and thrown down the center of the stairwell. At the top, he confronts the Bauta killer: Sarah Fabrizzi. She had been a member of Lodge Four, and killed the others both to cover up her tracks and consolidate a supernatural power said to be distributed by Ushuloda and spread amongst the Unhultians. Fabrizzi tries to kill Tadeo, but he shoots her and she falls off of the balcony and onto the cliff below with the other skeletons. Armstrong then shows up; he explains that he's not really Armstrong but a temporary form of the deity Ushuloda, and he has picked Tadeo to be his vessel. He also notes that Tadeo has proved himself so Ushuloda will let him choose his own fate. Tadeo climbs and stands on the balcony's railing as the film cuts to black. It is left ambiguous whether or not Tadeo jumps or if Ushuloda really exists.
