The film begins on a prison asteroid in outer space. A group of unseen creatures known as krites are set to be transported to another station. When the seemingly intelligent creatures hijack a ship away from the station, the leader of the station hires two transforming bounty hunters to hunt the little monsters down.
Meanwhile on the Earth we are introduced to the Brown family, living in a rural Kansas town: the father, Jay; the mother, Helen; the teenage daughter, April; and the trouble-making son, Brad. Brad often hangs out with dopey mechanic, Charlie. When he and Brad are outside, Charlie accidentally hits Brad's sister with a slingshot pellet and Brad, not wanting his friend to get into trouble, passes the blame on himself. He is grounded, but sneaks out onto the roof. While on the roof Brad sees what appears to be a comet streaking the sky, falling somewhere in the distance.
The comet, which in actuality is the Crite's ship lands causing the ground to shake, which alarms Jay and his wife. Walking outside to investigate Jay discovers his son, Brad, in the tree, who says he was thrown clear out of his window by the earthquake. Jay, obviously not believing this, takes his son along with him to find the comet.
At the landing site, subtitled, the creatures survey the damage, and set out for food. Devouring the majority of a steer, the Crites are interrupted upon Jay and Brad's discovery of the cattle's carcass. Shortly afterward the Critters make their way towards the farm and manage to cut the power. Jay, Helen, and Brad go to the circuit breaker where Jay is attacked by one of the Critters. The family manages to evade the Critters and get into the house. Meanwhile the two bounty hunters, already landed on the Earth, travel from pillar to post in the town looking for the Critters. When Brad goes for help he manages to bring the bounty hunters to his home. After their arrival the Critters flee back to their ship kidnapping April. Brad rescues April from the ship but while fleeing from the main Crite, accidently drops a large homemade bomb which he was unable to light. Charlie then gets the idea to light the bomb with a molotov cocktail created from the whiskey he's always carrying. The Critters lift off, and blast the Brown's house into pieces. Their ship explodes supposedly killing all the critters. The bounty hunters leave in their ship after giving Brad a strange remote-like device. The Brown's house is repaired in less than a minute, with a push of a button on the remote-like device. At the very end the camera zooms into the barn where three green eggs lay in the chicken-coop and begin to cackle, implying there will be a sequel.