A drifter, John Hall (David Andrews) goes to work at an old textile mill in Maine, where workers are disappearing. In the opening scene a guy working late on the ancient machines encounters the rats which infest the place. He's clearly used to them and even has names for a few. Then, more and more arrive-along with a much bigger creature, who remains off screen, and attacks him. He falls into the thrashing machine and they find his remains the next day. When John first starts working there he begins to think something is up-which is complicated by the textiles owner, Warwick (Stephen Macht), who is simply an asshole. We learn that there are some subterranean floors of the mill, which lead down to an underground river, and that a huge,previously unknown animal exists down there and has been feeding off the local graveyard. Now it's found a new food source.
I wasn't too crazy about this adaptation of the Stephen King story when it first came out, as the story is far better. But as a monster movie this holds up extremely well, as do the practical/mechanical effects of the giant bat-monster. It's far more effective than CGI and benefits the movie.