On an asteroid prison, a group of dangerous aliens known as Krites are set to be transported to another station. The Krites engineer an escape and hijack a ship, prompting the warden to hire two shape-changing bounty hunters, Ug and Lee, to pursue them to Earth. Studying life on Earth via various satellite television transmissions, Ug assumes the form of rock star Johnny Steele, while Lee remains undecided, thus retaining his blank, featureless head.




On a rural Kansas farm, the Brown family sits down to breakfast. Father Jay and mother Helen send teenage daughter April and younger son Brad off to school while waiting on mechanic Charlie McFadden. A former baseball pitcher, Charlie has become the town drunk and crackpot, with claims of alien abductions foretold by messages through his fillings.




That evening, Brad overhears the Krites' stolen ship crash-landing, and goes to investigate with Jay. They discover a cow that has been partially eaten by the Krites, and, disturbed, flee back to the house. Meanwhile, the Krites kill and feed on a local police officer (later, the bounty hunters take his police car as a method of transport), and later besiege the farm and cut its electrical connection. While checking the circuit breaker, Jay is attacked by one of the Critters and, despite being severely wounded, just barely manages to escape.




In the barn, April is rolling in the hay with her boyfriend Steve when he is attacked and killed by one of the Krites; the creature itself is slain when it devours one of Brad's lit firecrackers. The remaining Krites sabotage the Browns' and Steve's cars, forcing the Browns to shelter themselves inside their home. Meanwhile, the bounty hunters search the town for the Krites, causing a panic at the church and bowling alley, with the second hunter assuming the form of various townspeople, including Charlie. Brad escapes the farm to get help and runs into the bounty hunters, and upon learning of their true nature and intentions, he leads them to the Krites' location.




The remaining Krites kidnap April and return to their ship when the bounty hunters arrive, and attempt to flee. Charlie and Brad manage to rescue April, but Brad drops a large firecracker he intended to use to destroy the ship when the Critters discover their escape. Just as the Krites take off and destroy the farmhouse out of spite, Charlie throws a Molotov cocktail made from his whiskey bottle into the ship, causing a fire which detonates the cracker and kills the Krites. The bounty hunters leave in their ship after giving Brad a handheld device to contact them in case of future invasion, and also restore the house. Unbeknownst to them, Krite eggs can be seen in the barn inside a chicken's nest that seem to be ready to hatch.