Helen Quilley (Gwyneth Paltrow) is fired from her PR job, in the elevator a man, James (John Hannah) picks up her dropped earring, and she arrives to catch the tube. A little girl with a doll gets in her way making her a few seconds late and the doors close on her, time rewinds and we see what would have happened if the little girl is pulled away from Helen's path and she catches the train.
Helen who missed the train goes outside for a taxi and a mugger tries to run off with her handbag, she falls and hits her head, so the taxi driver takes her to Casualty. Meanwhile Helen who caught the train is being annoyed by the man sitting next to her, James. She eventually starts to talk to him, and apologises for being rude and heads home to find her boyfriend, Gerry, in bed with another woman. Helen storms out to a bar and gets drunk.
Helen who missed the train arrives home and doesn't catch her boyfriend in bed, so has to get two jobs to pay for her bills, whilst being certain her boyfriend is having an affair. Helen who caught the train goes to live with her best friend, gets her hair cut short and blonde (thus making her Blonde Helen), starts to date James and starts her own business whilst her boyfriend tries to win her back.
The movie is about how catching or missing the train affects Helen's life in very different ways.