The film opens with a voice over describing how the brain begins to shut down at an altitude of 20,000 feet, the human body unable to survive at such heights. A mountaineering group is shown climbing a cliff face in the snow, until an avalanche causes the rigging to break and they fall to their deaths.
Alice (Graham) is a young American woman from the flat lands living in London who believes she is happy in a secure job and a relationship with her live-in boyfriend. However, after a chance encounter with a mysterious stranger (Fiennes), she seeks him out and the two end up in the backyard having fast and sensuous sex. She returns home to her boyfriend, unsuccessfully attempting to bring out the same feelings between them she had with her stranger.
The following day she seeks the stranger out again, discovering his name is Adam - a mountain climber who is considered a hero after a tragic outing where one team lost their lives. She leaves her boyfriend and begins a relationship with Adam. He soon asks Alice to marry him, a proposal she happily accepts, but her newfound happiness is haunted by a series of mysterious phone calls and messages. On the day of their wedding, she receives an unmarked letter warning her against Adam. She ignores it and the two marry. Soon after she begins receiving more letters and begins to wonder about her husband's mysterious past. She becomes even more alarmed when she searches a locked wardrobe and finds a box of old letters from an ex-lover.
Alice searches after several leads as to who is sending the warnings and discovers a woman from Adam's past had gone missing. She suspects he had something to do with it and eventually runs from him. She goes to police but they cannot do anything without any evidence. Alice then goes to seek for help with Adam's sister, Deborah, only to find out that she was responsible for the death of his ex-lover. It is revealed that Deborah has an incestuous past with Adam, resulting in her subsequent possessiveness. As she tries to mete the same fate out to Alice, out comes Adam from the snowy perilous night to her rescue. Livid, he dismisses the past on account of their adolescence. Struck and fallen, Deborah makes an attempt on his life, stopped short by Alice's timely intervention, mortally wounding Deborah with a flare pistol. Adam admits to Alice that he and his sister were once together as children, and that he thought that if Alice just trusted him, then everything would be alright. Next morning, Adam is apparently taken away by the Police, betraying no hints of the possibility of their eventual union.
Two years later Alice and Adam see one another again on either side of an escalator - she's going down and he's riding up. They both watch each other as they pass without saying anything. Adam stops at the top and turns to stare back at Alice before walking away. The film ends with Alice's voice over recollecting the events in posterity and wondering what might have happened had fate not led her to Adam one morning. She wonders if the passion between them could ever have lasted, and that a girl born in the flatlands could never have survived at 20,000 feet anyway.