The narrator, William 'Bill' Davis, a Vietnam veteran returned home in 1971, takes work at Temp-O in Portland, Maine, where his prodigious stenographic skills (180 wpm) and eidetic memory make him their best transcriber. Emotionally numbed by the war, he answers a newspaper ad seeking a 'phlegmatic' assistant for a 'Gentleman Scientist.' He drives to Castle Rock and meets Elgin, a wealthy, oddly serene man living on Dark Score Lake, who hires him to assist with secretive experiments on dreams—documented only via stenography and Polaroids, with no audio or film.

Elgin tests twelve subjects recruited via the book of photographs Bill is given. Each subject drinks a 'light hypnotic' (diluted Flurazepam), looks at a picture of a red house with a green door, and is hypnotically instructed to enter, kneel in the living room doorway, and lift up the floor. The first subject, Althea Gibson, dreams of her grandfather's house and lifts the floor partway, smelling stench; her teeth grow grotesquely during sleep, captured by Polaroids. Elgin reveals his true ambition: not to study dreams but to peek under them at a binding cosmic reality.

Subsequent runs yield mixed results until Hiram Gaskill writes Vietnamese phrases in his sleep—'the moon is full of demons'—a message Elgin says was meant for Bill. Bill warns Elgin to stop, but his own reawakening curiosity keeps him on. Annette Crosby wakes screaming from the green door dream. Then Burt Devereaux, a math teacher, suffers a horrific reaction: his eyes swell, blacken, and split, releasing black tendril-like entities that drift toward the one-way glass. Bill restrains Elgin from entering. Devereaux is left a breathing, empty husk.

To cover up the disaster, Bill drives Devereaux to a rest area on Route 119, props him in the driver's seat, and walks away, hitchhiking back to Castle Rock with a Korean War veteran. Returning to the lake house, he finds Elgin on the testing couch with a black filament-sack engulfing his head; the tendrils on the glass spell out 'WILLIAM DAVIS.' Bill turns on the gas stove and oven, blows out the pilots, takes Elgin's Zippo, retrieves his belongings, and ignites the house in the rainstorm.

Bill returns to Portland, rehires at Temp-O, and reads of Elgin's death—ruled possible suicide. Police question him; he gives a clean account, and the investigation goes nowhere. By September he drifts to Nebraska, working on a farm. Years later, he writes this confession during a blizzard, haunted by dreams of the red house with the green door, knowing that one day he will open that door, kneel, lift the floor, and find no mercy.