"The Aliens films meet Event Horizon" is a pretty solid - if intrinsically derivative - elevator pitch, and the producers of this film probably had high hopes for a franchise. As it was, this film didn't do all that well financially. Possibly because it's not terribly good.
A man awakes from 'cold sleep' on a darkened spaceship. He stumbles around, trying to get his bearings, confused by the semi-derelict state of the ship, the fact that there was no-one to meet him when he awoke, and the memory-loss we will later discover affects everyone who spends an extended period in stasis.
The man eventually pieces together who he is - Bowen, a flight mechanic - and awakens his superior officer, Lt Payton. The two men find that all the doors out of their area are inoperable, but Payton locates a still-working console from which he can hopefully guide Bowen through the service ducts and into the rest of the ship.
While crawling through the ducts, Bowen has a panic attack. Payton talks him down from it, and then then the two exchange one off the clumsiest bits of exposition I've ever seen when they discuss 'Pandorum' a psychosis that afflicts those who spend an extended period in space, and which Bowen secretly fears he is developing. It's a shame that having set up a good reason for exposition (newly awakened characters who don't know what is happening, and have fragmented memories besides), the script does such a bad job of actually executing it.
Bowen will soon have bigger things to worry about than his mental health, however, as it emerges that large parts of the ship are now the domain of freakish-looking creatures with a taste for human flesh. Also, the ship's reactor is on the verge of meltdown, which is probably why he was awakened.
Our possibly psychotic hero must travel the length of the ship, dodging monsters and trying to reason with the few human survivors he meets - people who are understandably not in the friendliest of moods - to save the ship and its cargo: fifteen thousand human colonists, locked in cold sleep.
This is not a terrible movie, but it fails to generate much real tension (at least partly because the script has a habit of having Bowen fall off or through things with such frequency that I began to snicker every time it happened), and it doesn't offer much to make it more than its derivative concept. It doesn't go off the rails as badly as I thought Event Horizon did, but its rails aren't as interesting to begin with.
If SF/horror fusion is your thing, then you might want to check this out. Otherwise, you can skip it.