The plot: a shallow high-society party hopper played by Franke Potente falls asleep in the station, misses the last subway home and gets trapped in the London Underground. There's something down there with her, something that really likes killing people. She meets up with various people who would be in the Underground after hours (guards, the homeless, workers), there's screaming, gore, etc.
This is a by the numbers monster thriller. The scares and starts are plentiful, and the director earns them. The rules are followed (if someone dies horribly because of your cowardice, you're gonna get it). The first time we actually get a good look at the monster is handled masterfully.
My only complaint is that there are some plot holes that ruined my suspension of disbelief, holes that seem to be there through laziness or inattention rather than any insoulable narrative problems. For example, early in the film, a character shows up somewhere he couldn't possibly be, just so he can scare the protaganist, then die horribly.