Odie becomes a giant green prehistoric dog after he comes in contact with a meteorite.
Jon is watching the stars up in the night sky. Garfield and Odie want him inside to make dinner, but before he does, they notice a shooting star.
Meanwhile, Professor Bonkers becomes aware of the shooting star at the observatory. The professor figures it to be the meteor Beta Blue, which is said to have the power to turn others into earlier forms during contact. This is later proven when, after it lands on Earth, it turns a man into a caveman and a modern car into an automobile model from the early 20th century.
At Vito's, the meteorite affects a fly (which Vito had been trying to get rid of) and Odie (who has it stuck to his paw after stepping in gum). The police are brought over to deal with a giant fly, only to find a normal fly when they arrive. Afterward, however, Garfield and Jon notice a humongous, prehistoric Odie before them. Odie then causes terror around the city, with the police and Al the dogcatcher trying to deal with him.
Garfield and Jon then take control of an excavator to bring Odie out of the city and into the forest, using a tree as a lure. Al looks for Odie in the forest, figuring it should be easy, only to find out how big he's become. While fleeing from the giant dog, he winds up getting the meteorite on his nose. Odie returns to his normal self, with Garfield and Jon bringing him back home, while Al becomes a caveman, attacking his own vehicle.