Newspaper photojournalist Audrey Ames (Peggie Castle) stumbles upon the town of Ludlow in Illinois, which has been inexplicably destroyed with all its population missing presumed dead.
She travels to a nearby experimental farm run by the Department of Agriculture where she meets Dr Ed Wainwright (Peter Graves), a scientist who is experimenting with the use of radiation on fruits and vegetables to make them larger and address the world hunger problem.
Unfortunately, locusts have feasted on all the radioactive wheat stored in a nearby grain silo and now an army of giant grasshoppers now threatens Chicago.
Wainwright and Aimes meet with General Hanson (Morris Ankrum), Colonel Sturgeon (Thomas Browne Henry), and Captain Barton (James Seay) to devise a solution. Machine guns and artillery are ineffective against the creatures and there are far too many to effectively deal with all at once.
The Army and National Guard are called upon to help protect the city but the monsters quickly invade Chicago and begin to feast on human flesh as well as several buildings.
General Hanson decides the only way to destroy the beasts en masse is to use a nuclear weapon and destroy Chicago. However, Dr Wainwright realises that the locusts are warm-weather creatures and concludes they might be able to lure the locusts into Lake Michigan. There, the cold water will incapacitate them, and they will drown.
The lure itself will be a decoy locust mating call, generated electronically with test-tone oscillators. The plan is put into effect, and it works at the last possible moment – the monstrous locusts drown.
Over 200 grasshoppers were used for the movie. During the filming, they began to eat each other and by the time the last shots were done, only a dozen were left.
Dr Ed Wainwright
General John Hanson
Colonel Tom Sturgeon
Thomas Browne Henry
Captain James Barton
General John T. Short
Bert I. Gordon