Marie (DEMI MOORE) is an American literary critic living in southern France with her two daughters Jennifer (ELOISE EONNET) and Sarah (CHAYA CUENOT). A widow of two years, Marie confides in her best friend, Jessie (SINEAD CUSACK), and therapist, Dr. Langer (JOSS ACKLAND), that she has such vividly realistic dreams of living a separate life in New York that she can't differentiate reality from dream.
Marty (DEMI MOORE), her Manhattan-based doppelganger, has the same dilemma. A successful, but single and childless literary agent, Marty leads a happy life save for the knowledge that when she goes to bed in the Big Apple, she awakens the next morning in Province, a dilemma she also discusses with her New York therapist, Dr. Peters (PETER RIEGERT).
Although the dreams don't incapacitate the protagonist, when she's either Marie or Marty, to any great extent, they do have her wondering which world is real. Things then become more complicated when Marie meets William (STELLAN SKARSGÄRD), a novelist whose earlier work she savaged. Despite that, William and Marie's kids take an immediate liking to one another and he and Marie soon begin a romance.
Back in New York, Marty has started seeing Aaron (WILLIAM FICHTNER), a laidback accountant, and while initially reluctant, Marty eventually falls for him. As Marty repeatedly follows the pattern of falling asleep one night and waking up the next morning as Marie, and then her doing the reverse the next morning and night, and with both being conscious of the other's world and new romance, the confused woman must figure out what's real and not before she loses everything in both worlds.