Helen (Gwyneth Paltrow, from A Perfect Murder and Shakespeare in Love) is having said worst day of her life. She's barely in the office one morning when she finds herself fired from her job at a London PR firm. With nothing else to do, she reluctantly heads for home. In the tube station, she just misses a train -- and another Helen just makes the train. At the sliding doors of a London Underground train, the universe splits in two, and we watch the lives of both Helens unfold.

It's a great concept. Writer and director Peter Howitt knows this is the kind of thing we all wonder about: Who did I miss meeting when I decided to walk down this street instead of that one? How would my life be different if only I'd taken the road not taken? Howitt favors an inevitability, a brand of destiny about who we meet and what we do -- his Helens converge again, as if she were fated to end up how she ends up no matter what she did -- but whether you agree with that belief or not, it makes for an engaging story.

The Helen who makes the train meets James (John Hannah), a self-described "rather annoying chap" who's prone to quoting Monty Python inappropriately. Helen's only interested at the moment in reading her book, but she and James will cross paths again. When she gets home, she finds her boyfriend, Gerry (John Lynch) -- whom she's supporting, the lout, while he writes his first novel -- boinking Lydia (Jeanne Tripplehorn), and Helen immediately removes herself to her friend Anna's (Zara Turner) house. The Helen who misses the train, however, never meets James and is delayed long enough to also miss running into Lydia, and she continues her relationship with Gerry, unaware of his two-timing.

As the two Helens diverge, there are some interesting and amusing commonalities: both Helens have hangovers the morning after they get sacked, for very different reasons, and both end up working with restaurants, though in very different ways. And you'll find yourself wondering, as you follow the Helens around, how often you meet people to whom you have a connection you're unaware of.

My favorite touch in Sliding Doors, though, is the spot-on depiction of the differences between men's relationships with their best friends and women's relationships with theirs. The opposite ways in which Anna and Gerry's mate Russell (Douglas McFerran) react to their friends' troubles is all the more hilarious for the ring of truth about it.

What makes Sliding Doors work, when it comes right down to it, though, is not the clever concept but the great cast. Paltrow is rapidly becoming one of my favorite young actors, with her combination of strength and vulnerability, and the John Cusack-esque Hannah is every bit as winsome here -- albeit in a totally different way -- as he is in The Mummy (in which he plays the twit brother).

Sliding Doors is a really fun little movie.