A copper refinery owned by Thor Carlsson operates trains that pass by a young boy's home—a quiet detail that lingers as part of a family history tied to the rails. Decades later, in 1987, Dorothy Carlsson, Sean Young, a University of Pennsylvania student and heiress to the Carlsson fortune, doodles images of her wedding during class. Preparing to meet someone at City Hall, Dorothy meets her friend Patricia Farren, but denies that she is seeing a “mystery man.” At City Hall, Dorothy meets Jonathan Corliss, Matt Dillon, her secret boyfriend. When the marriage license bureau is closed for lunch, Jonathan takes her to the building's rooftop, where he suddenly pushes her to her death. Before leaving, he takes her engraved lighter and mails a letter she had written. Her body crashes through the skylight and lands in the lobby below.
Dorothy's apparent suicide shocks her father, Thor Carlsson, Max von Sydow, and her identical twin sister, Ellen Carlsson, Lachele Carl. Discovering that Dorothy was pregnant, Thor concludes her death was a suicide brought on by shame. Ellen, however, is skeptical. Jonathan, meanwhile, returns to Pittsburgh, where he lives with his mother and clippings about the Carlsson family. Promising to make something of himself, he hitchhikes to New York and assumes the identity of Jay Faraday, a drifter whose parents had died on Korean Air Lines Flight 007.
Four months later, Ellen works at Castle House, a shelter in New York. Still investigating Dorothy's death, she contacts Detective Dan Corelli in Philadelphia. She presents Dorothy's wedding sketch, dated the day of her death, and suggests that she was murdered by a boyfriend. Corelli remains unconvinced.
Ellen discovers Dorothy had dated another student prior to her death—Tommy Roussell. When Ellen and Tommy reconnect, he recalls Dorothy had begun dating someone else shortly before her death. While Tommy searches for the man's photo in an old yearbook, Jonathan stalks Ellen from his car. Before Tommy can show Ellen the photo, Jonathan strangles him, forges a suicide note on Tommy's computer, and stages the scene as a suicide confession. Ellen, unaware of Jonathan's deception, believes that Tommy was Dorothy's killer.
Ellen returns to New York and reunites with her boyfriend—now going by Jay Faraday, Matt Dillon, Jonathan's assumed identity. They grow closer while working together at Castle House, and Jay impresses Thor with his drive. Ellen and Jay marry, and he joins Carlsson Copper. When Patricia remembers the identity of Dorothy's former boyfriend and plans to visit Ellen, Jay intercepts the call. He strangles Patricia in her hotel room, dismembers her body, and dumps it in the East River—all while maintaining the appearance of a normal evening with Ellen.
Later, a detective investigating Patricia's disappearance informs Ellen that her contact information was found in Patricia's diary. Growing suspicious, Ellen verifies that Tommy had been institutionalized at the time of Dorothy's death and again approaches Detective Corelli, though the case remains closed.
While at a bar, Jay is recognized by an acquaintance as Jonathan Corliss. Though he denies it and attacks the man, the incident unnerves Ellen. She consults a university yearbook and confirms the resemblance. Tracing Jonathan's past, Ellen visits his mother in Pittsburgh, who claims that her son died by suicide, though his body was never found. Inside the home, Ellen finds a suitcase filled with newspaper clippings about her family and Dorothy's lighter.
Jonathan arrives and confesses everything: he killed Faraday and assumed his identity to marry into the Carlsson fortune. Dorothy's pregnancy had threatened his plan, prompting him to kill her. As he attempts to murder Ellen, she escapes to the train tracks outside. During the pursuit, Jonathan is struck and killed by a Carlsson Copper train—at the same spot where he once watched the trains as a boy.
