(Spoilers ahead --- then again, have I ever written a review in which there weren't?)
In both Robocop and Buckaroo Banzai, Peter Weller was surrounded by a superb supporting cast; so it's easy to overlook how talented he is on his own. In Of Unknown Origin, however, he's largely a one man show, and he's wonderful.
He plays Bart Hughes, an up-and-coming New York executive on the verge of becoming a partner at his firm. His wife (Shannon Tweed -- yes, we get to see her boobs) and son go on vacation for a month. So he's alone in his brownstone. An important thing to note about the brownstone: Bart has refurbished it completely on his own, by hand. He takes great pride in this and points it out to visitors throughout the movie.
Shortly after his wife and son leave, he begins to notice signs of a rat infestation. It's only one rat, but it's a doozy. We don't get a sense of its full size until late in the movie, but it's as big as Bart's head. It keeps Bart up all night with its scratching claws. It chews on all kinds of important things around the house, and it sidesteps Bart's every attempt at catching it. Traps sit unsprung around the building, poison goes untouched. He calls for an exterminator to take the little critter down while Bart's at work and it chews up the check for the extermination fee.
His coworkers notice he's becoming slightly unglued. One of my favorite moments happens when his secretary drops by the brownstone to check on him. She calls into the house from the street and he bellows from deep within the house, "Leave us alone!" Us. The rat may be his enemy, but only Bart and the rat know the lengths to which their vendetta runs.
He grows more desperate the longer the rat eludes him, but even in his desperation, he never quite loses his grip on reality. The best thing about this movie is that Bart keeps his perspective the entire flick. The rat is obviously annoying him to bejeezus and back, but this isn't that clichéd story about the protagonist being tormented into insanity. Even through the torment at home, he continues to show up to work. He does his job well up until his frustration reaches its peak. When it does, he simply goes to his boss and points out plainly that he's got a problem, he's going to deal with it, and when it's done, he'll come back to work. He also symbolically makes the same move with his wife by not answering the phone when she calls. It's not because he's lost his shit; he's just locked into a battle of wills with the rat and can't be distracted by anything else.
The movie has some nice adornments. For one, it uses dream sequences well: the best involves the wife and son coming home and the son, unattended, pouring himself some cereal and then unwittingly scooping rat poison onto it -- and this is some heavyweight rat poison, designed to make a rat so thirsty it'll leave the house it's infesting and crawl into the sewer to drink water until it bursts.
Another nice touch is all the shit-talking Bart does to the rat. At one point Bart warns the rat, "I've got friends in Jersey."
And the final confrontation is a hoot just for the sheer volume of stuff that gets broken in the process. Pipes get snapped open; walls, mirrors and furniture get smashed with Bart's homemade mace. He manages to kill the thing just before his wife and son arrive home and as he's walking towards the door, Bart takes a perfectly intact vase and tips it casually off of its pedestal, letting it smash on the floor. Because, y'know, whatever, by that point.
I would absolutely recommend this for any fans of Peter Weller or anyone looking for a double feature with Willard. Great fun.