In 1963 Cedar Hills, local police officer Sam Green (Tobi Bakare) arrives at the Cedar Steel Mill to search for a missing child. Sam makes his way to the furnace chamber of the mill where he discovers mill worker Frank Stone (Matt Mordak) about to throw the kidnapped infant into the furnace. Regardless of whether or not he is quick enough to save the child, Sam shoots Frank dead. In 2024, Madison Rivera-Platt (Diana Bermudez) travels to Gerant Manor in England after being invited by collector Augustine Lieber (Hannah Morrish). On the way, she meets Linda Castle (Lucy Griffiths), a filmmaker, and Stan (Andrew Krueger), a self-proclaimed businessman.

Stan remarks he knows of Linda from Murder Mill, one of her films which is supposedly cursed. Augustine arrives and explains she has invited the group to purchase the reels of Murder Mill that each one of them owns. The group decides to explore the manor after she leaves. They find evidence of high-tech machinery, numerous grim artifacts from history and fiction, and literature related to Frank Stone. They also learn how each of them got their film strips, with Madison inheriting hers from her mother Bonnie (Bermudez) who recently died and whom Linda once knew back in Cedar Hills, and Stan getting his from a "Robbie G." who is now dead.

At that moment, Sam Green suddenly appears. In 1980, Linda is making Murder Mill with Jaime Rivera (Andrew Wheildon-Dennis) and his girlfriend Chris Gordon (Rebecca LaChance). Sam, now the sheriff of Cedar Hills, discovers the group shooting in the now condemned steel mill and orders them to leave. With their camera broken in the confrontation, Linda explains Sam's connection to Frank Stone as well as Frank's kidnappings and murders to Chris, who is not originally from the town. She is inspired and becomes determined to get back in the mill to get shots related to Frank.

They head into town to fix the camera, meeting Sam's son Robert (Idris Debrand) who says he can get them back in the mill, and eventually get a replacement from a mysterious shopkeeper of a curiosities store. The group sneaks back into the mill to finish the movie, with Jaime's sister Bonnie tagging along. As they explore to get footage, Chris captures strange paranormal energy using the camera. They then discover a room that contains missed evidence of Frank's crimes and even the remains of some of his victims. Robert hurries to fetch Sam while Chris and Jaime wait for him to return. At that moment, the energy surges through the mill and the undead form of Frank Stone manifests before them, unable to be seen visibly without the camera.

Chris is suddenly dragged into a portal that opens nearby and disappears while Frank hunts the rest of the crew through the mill, possibly killing Jaime. Back in 2024, Sam accuses Stan of taking his filmstrip from a troubled adult Robert while explaining he is there to stop them from selling the reels to Augustine, though is unable to explain the details of why clearly. Madison and Stan explore further, discovering a gigantic chamber containing a machine called the Horologium. Chris suddenly emerges from the device, apparently having travelled through time. Depending on player choices, she can either immediately return to 1980 or remain in 2024.

Back with Linda, Sam confuses matters by recalling memories that don't match Linda's and she discovers an archive containing alternate film reels of Murder Mill, along with objects that don't match what she remembers shooting. After Sam finds her again, Linda remarks he should be much older than he actually is and accuses him of not being the Sam Green she knows. After the group reunite in a theatre room, a reel of Murder Mill begins to play which shows the concluding events of 1980.

Frank's spirit pursues the remaining crew of Murder Mill and Bonnie is killed being impaled on a hook. Sam then arrives with Robert after the survivors have fled into the storm drains underneath the mill. Using the camera to ward off Frank, Sam can save Jaime and Chris if they have survived until this point before confronting Frank going after Linda. Otherwise, he and the other survivors can die. In the end, the survivors are able to use the camera to trap Frank's spirit inside the film itself, splitting up the film among themselves to ensure Frank can never return. It becomes clear that the version of Murder Mill the group in 2024 is seeing does not match their own personal histories.