A necking couple in a car is horrified by something unseen, and then after the credits their smashed vehicle is discovered. In the process of investigating that, we're also told (but not shown) that an Illinois town has been demolished, all its residents missing. ("Something happened off-screen here!") The National Guard sets up roadblocks, but the feisty female reporter Audrey Ames isn't deterred. She happens on the agricultural research lab of Ed Wainwright (played by Peter Graves, who was often imitated when he appeared in several other Mystery Science Theater episodes), who's growing gigantic tomatoes and strawberries via the aid of radiation. His assistant Frank Johnson has been rendered deaf and mute by a mishap with that radiation, but neither of them are troubled by it, until they head to a demolished grain storage warehouse and discover a giant (yet poorly matted-in) grasshopper... ("Quickly! Get a really big applesauce jar with holes punched in the lid!")
With Frank devoured, the National Guard steps in, but doesn't have much luck. Ed Wainwright heads off to Washington, D.C. to warn of the dangers of regular-sized locusts (using a movie to do so is rather reminiscent of Them!), but in the meantime the grasshoppers have broken through the Army lines ("Even our best footage can't stop them!") and destroyed a whole series of Illinois towns, again off-screen. With the grasshoppers closing in on Chicago, Ed manages to capture one of them to, as he explains in one more of his dodgy scientific monologues, find a signal that will attract them straight into Lake Michigan. In the meantime, the Army is preparing to drop an atomic bomb on Chicago, but in the nick of time (even as grasshoppers scale pictures of Chicago buildings), Ed finds his signal. Holding off the grasshoppers with his very own machine gun ("Pete, Pete! Short, controlled bursts!"), Ed survives until the grasshoppers flood into the lake, and winds up paired off with Audrey. ("Well, we just spent the goofiest week of our entire lives.")
Just as Dr. Forrester promises, the movie and perhaps by extension the episode seem a bit slow to get started; when the grasshoppers are actually on screen, though, things seem livelier. The "host segments" include Mike deciding to "call the Mads" and getting a strange glimpse into their personal lives, Crow presenting his screenplay about Peter Graves at the University of Minnesota, and Dr. Forrester and TV's Frank following up on being discovered by indulging in a manly boxing match.