At his mountain cabin, chemistry teacher Barney Cousins prepares to abduct a woman using a handkerchief soaked in chloroform. Elsewhere, Jeff Harriman and Diane Shaver, a young couple from Seattle, Washington, drive through Mt. St. Helens on vacation. Jeff’s car runs out of gas in a highway tunnel, and Diane panics as a truck nearly runs them over. The couple argues, and Jeff sets out alone to get gas. When he returns, they drive to a Mini-Mart where Diane makes Jeff promise to never abandon her again. She offers to drive the next leg and takes the car keys with her when she goes inside to use the bathroom. Jeff awaits her return, but becomes worried after several minutes have passed. He searches for Diane, to no avail. Later, he reports her missing and loses his temper when a policeman suggests Diane left of her own volition. Jeff spends the night in the parking lot, smoking Diane’s discarded cigarette butts and burying his face in her sweater. Three years later, Jeff has not given up his search for Diane. At the school where Barney Cousins teaches, Barney’s colleague points out a new missing persons poster for Diane and remarks that Jeff posts a fresh sign every three months. Barney admires Jeff’s perseverance. One night, an exhausted Jeff goes to a diner and meets Rita Baker, a feisty waitress who recognizes him from high school. Rita takes an interest in Jeff and encourages him to get some rest on a cot in back of the diner. Sometime later, Rita moves into Jeff’s apartment. Riffling through his things, she is disturbed to find a loaded gun, but Jeff claims he needs it for protection. After submitting his novel to publishers, Jeff takes a meeting with Arthur Bernard, an editor who turns down the book but commissions Jeff to write one about his search for Diane, instead. Although he has tried to put Diane’s disappearance behind him now that he is with Rita, Jeff agrees and begins work on the book. Rita is curious about Jeff’s new project, but he refuses to discuss it. Pretending to join the Army Reserves, he secretly sets up an office at a nearby motel. One day, Rita snoops around on Jeff’s computer and discovers his scheme. She surprises Jeff at the motel and finds his room plastered with newspaper articles about Diane’s disappearance. Rita takes Jeff back to the Mini-Mart where Diane went missing and tells him he must choose between her and his obsession with Diane. Jeff admits that not knowing what happened to Diane has nearly driven him crazy, but he promises Rita he loves only her, and agrees to give up writing the book. Sometime later, Barney Cousins notices that Jeff has stopped his search. Barney types a letter to Jeff, arranging to meet him at a yacht club. Jeff goes to the appointed meeting spot, and has Rita meet him there. Barney observes from afar as Rita reprimands Jeff. She refuses to believe that the letter came from Diane’s actual kidnapper and urges Jeff to hand the letter over to police. Jeff insists on pursuing the man himself, and Rita storms away. At their apartment, she packs her things to leave. Barney sneaks inside with a chloroform-soaked handkerchief, but Rita leaves before he has a chance to attack. When Jeff returns home, Barney introduces himself as Diane’s captor. Livid, Jeff kicks him down the stairwell outside his apartment. Barney offers to show Jeff what happened to Diane, insisting that Jeff drive away with him. Jeff reluctantly agrees, and Barney takes him back to the Mini-Mart. He offers Jeff a drugged cup of coffee, saying that Jeff must go through exactly what Diane went through if he wants to find out what happened to her. Despite his misgivings, Jeff drinks the coffee. Meanwhile, Rita gets drunk at a bar and calls home. She hears the new answering machine greeting Jeff was leaving when Barney introduced himself. Hearing Barney’s voice, Rita panics and rushes home. She questions her mentally ill neighbor, Miss Carmichael, who saw Jeff leave with Barney. She recalls Barney’s license plate number from memory. At the police station, Rita falsely reports that her car was stolen, using Barney’s license plate number to ascertain his last name and home address. Outside Barney’s house, Rita sees Barney’s thirteen-year-old daughter, Denise, who believes her dad is having a romantic love affair and excitedly assumes Rita is his mistress. Rita goes along with the ruse and tricks Denise into giving her directions to Barney’s cabin. Jeff wakes up in a coffin and panics as he realizes he has been buried alive. Rita arrives at the cabin. Outside, Barney creeps up behind her, but she knocks him over the head and runs through the woods and into the cabin, where she calls police. However, the telephone receiver is covered in chloroform and she drops it in a stupor. Barney corners Rita and offers her a cup of drugged coffee. He tells her if she wants to know what happened to Jeff, she must experience exactly what he did. Rita notices wood chips on Barney’s shoes and a shovel leaning against a wall, and recalls a mound of dirt outside where Jeff might have been buried. She convinces Barney that she kidnapped Denise and tells him he must drink the coffee to find out what happened to her. Barney complies and passes out. Rita runs outside with the shovel and digs up Jeff’s coffin. She finds him alive but unconscious just before Barney comes up behind her with a saw. Barney wrestles Rita to the ground. Gasping for air, Jeff regains consciousness and uses the shovel to deliver a fatal blow to Barney’s head. Jeff and Rita embrace, and Jeff assures her, “It’s over.” Sometime later, Jeff and Rita have lunch with Arthur Bernard, the book editor who earlier commissioned a book about Diane. Although Rita tells Bernard that she and Jeff want to put it behind them, Bernard insists that Jeff write about their horrific real-life experience just as a waiter delivers two coffees, and Rita and Jeff wave him away.