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Matthew (Josh Hartnett) and Lisa (Diane Kruger) are madly in love. But one day Lisa vanishes. Simon enters an emotional tailspin and begins an obsessive quest to find her. Years later, Simon's life has stabilized, and he's in a new relationship. Then he thinks he sees Lisa in a restaurant and fixates on her all over again. He snoops around and eventually tracks down the mysterious woman from the restaurant. Her name is Alex (Rose Byrne), but she acts just like Lisa. Wikipedia