Matheson, whose career in print, television, and movies now spans 60 years, is always the best adaptor of his own work, as The Omega Man or the Will Smith I am Legend prove through Matheson's absence. Pretty much everything Spielberg put on the screen in Duel was already there in Matheson's novella, which Matheson adapted for Spielberg's career-starting television movie.
Here, Matheson gives us an improbable tale that works because the hero's concerns can be applied to any number of real-world situations without losing the specificity and unique weirdness of the hero's plight. Scott Carey gets doused with some combination of insecticide and radiation. He starts shrinking. And he doesn't stop. Questions of masculinity, identity, and existence itself come into play. A media circus gathers outside Carey's door. And then there's that damned cat...and later, a very hungry spider.
A voiceover added to the end of the movie to make things a bit clearer (or at least more clearly hopeful and redemptive) doesn't damage the film too much, though that concluding sequence really could have remained mostly silent. There may be deep thoughts here, but Matheson keeps things moving, keeps things light at points, and writes several terrific action sequences. The visual effects are very good for their time or really any time. And remember, kids -- check your water heater regularly. Highly recommended.