Every morning, Christine Lucas (Nicole Kidman) wakes up terribly confused, with no idea who or where she is or who the strange man in her bed is. And every morning she sees a montage of photos on the bedroom wall telling her that the man is her husband Ben (Colin Firth). Viewers watching the first few minutes of Before I Go to Sleep might be similarly confused themselves and think they are seeing a Masterpiece Theatre remake of the Adam Sandler/Drew Barrymore comedy Fifty First Dates, but for two major differences. First, most women would much rather wake up next to Firth than Sandler, and, second, Before I Go to Sleep is a decent thriller instead of the usual insipid Sandler mush.

Both Sleep and Dates (as well as the superb Memento) use as a plot device a rare medical condition known as anterograde amnesia, in which the mind’s ability to create new memories is impaired. In Sleep, Christine’s mind erases all her memories of the previous day when she goes to sleep each night. As a result, she remembers nothing for the last several years, ever since she was injured in an accident. As Ben explains for what is undoubtedly the thousandth time what to Christine what happened to her, she calms down, until she receives a phone call from a Doctor Nasch (Mark Strong), a psychologist who tells her he’s been treating her for several months. At his suggestion, she finds a camera that he tells her is hidden in her closet and starts watching the video on the camera, which consists of a series of observations of the previous day that she’s recorded the night before. By watching those clips, she can know what she’s experienced previously. The last entry, from the night before, is chilling—on the clip, the recorded Christine says her life is in danger.

Sleep is based on a best selling novel by S.J. Watson, and the movie illustrates the difficulty involved in adapting certain works for the screen. The novel is told in the form of daily entries in a journal Christine keeps, and it begins with rather mundane descriptions of daily life before slowly morphing into a thriller as Christine realizes something is wrong. Screenwriter/director Rowan Joffe can’t take such a leisurely approach in a movie that’s barely 90 minutes long; so, instead, both Christine and viewers are thrown into a puzzling mystery from the moment the movie starts. This leads to a rapid fire sequence of events in which Christine falls into a pattern of visiting Doctor Nasch, who gives her more information about her condition every day, followed by her learning something new day (thanks in part to flashbacks she has). Most of these new revelations involve a twist of one sort or another, some of them rather unlikely.

Christine’s efforts to figure out who is a threat to her is made considerably easier by the fact that there’s a dearth of possible suspects on whom to cast suspicion. The only possible villains can be Ben and Dr. Nasch. The movie does its best to muddy the waters somewhat by making sure that both characters appear quite suspicious. For a psychologist, Nasch’s treatment methods seem very odd. He skulks around with Chrstine on their daily “therapy” meetings, often spending lots of time talking to her in dimly lit parking garages and having to restrain himself from getting too familiar with her. Ben, on the other hand, seems to neglect telling her a number of details from her past life, such as omitting to ever mention a good friend of hers named Claire, who eventually appears in the film to provide some needed exposition. However, whenever Christine confronts either Ben or Nasch about his peculiar activities, each provides a plausible reason that Christine quickly accepts.

In order to conclude the movie in 90 minutes, Joffe sacrifices a good bit of suspense and plot coherence. Events seem jumbled, even more than what would naturally result from Christine’s condition. The best amnesia thrillers take their time and typically have a hypnotic feel to them (like Hitchcock’s Spellbound). Sleep, on the other hand, often feels as rushed as some of the video clips Christine watches on her camera. When the villain finally reveals himself, it happens so abruptly that viewers may not believe what they’ve just seen.

Sleep does maintain enough of the intrigue of its original premise to be reasonably entertaining, thanks in large part to a strong performance by Kidman. She plays the harried woman in distress well and doesn’t engage in over-the-top histrionics. The film is also helped by the good chemistry she has with Firth, with whom she costarred earlier this year in The Railway Man. The movie requires Kidman to accept her relationship with Firth, over and over again, despite somewhat sketchy documentation of their marriage, and the chemistry they share makes such a leap of faith a bit more credible.