Helen Quilley works in PR for a big London company...at least, for the first five minutes of the film. After she is sacked for taking some of the boss's Smirnoff, she heads back down to the Underground to catch the train to take her back to her flat. But as she runs down, we suddenly see her life split off. In one version, she catches the train and in the second, she misses it. Throughout the rest of the film we see what would have happened in each scenario. Her whole life changes in that split second. When she catches the train, she meets a charming man called James and gets home to find her boyfriend Gerry in bed with another woman. When she misses it, she ends up getting mugged before landing a crummy waitress job.