Bart and his family have purchased a house they can't really afford and spent a lot of money they don't really have renovating it. It's all good, though: Bart's about to get a big promotion at work ... probably. Maybe. He buckles down to complete the project that will get him this promotion while his wife and kid head off on a little vacation. Suddenly ... a huge rat! The traps Bart sets can't kill it. The poison Bart lays just p**ses it off. The feral cat Bart turns loose in his house can't kill it (in fact, the rat makes a point of positioning the cat's carcass for maximum shock value). All that time Bart devotes to killing the rat sure makes it hard for him to get any work done on his big project. By the end of the movie, Bart is facing the prospect of divorce, the loss of his home and job, and maybe even the loss of his sanity. All because of a rat. This being a movie directed by George P. Cosmatos during the 1980s, it's obviously time for Bart to man-up and take the fight to the rat directly, like a taller, skinnier, less greased-up Rambo. Thing is, though, that this rat really is as big and dangerous as Bart's delirious ramblings have made it out to be ...
Of Unknown Origin probably isn't as good as I've made it sound but it is good. Peter Weller, as Bart, proves that he actually can act when he feels like it. The scene where he ruins his boss's dinner party by, apropos of nothing, spouting off scary rat trivia is nothing less than brilliant. Similarly, George Cosmatos proves that he actually can direct worth a damn. For one thing, the movie is perfectly paced. A lot of stuff happens and exactly enough time is given to all of it. Nothing drags, nothing ends too soon. Cosmatos gets a lot of mileage out of close-ups of the rat touching Bart's food, watching him in his sleep, or doing more disgusting stuff like rubbing its anal gland all over Bart's living room table. Cosmatos does a good job with the little things, too, such as Bart's commute to work becoming progressively louder and uglier as his struggle with the rat drags on.
Much of the horror of this movie comes from the fact that Bart - a man successful in every way modern man can be - can't come to grips with a rat. Sure, it's a big rat, but it's still just a rat. All it takes to completely undo Bart as a human being is for a rat to chew through some dishwasher hoses and a couple of wires, gnaw on a few of his food boxes, and p**s on his furniture. It must be significant that it is only when Bart confronts the rat on almost purely animal terms that he has even the slightest hope of defeating it. He may not be able to outsmart it, but he is a lot bigger and stronger than it is.