Garfield insists on his traditional birthday treat, a portion of lasagna for every year. Because Odie got one lost, John, who is out of ingredients, rushes out to get more. Then lands a spaceship with an alien race resembling lasagna with limbs. They deem earth suitable for colonization provided they can handle the only apparently dangerous creature: the orange cat Garfield.
Once more, Garfield locks out Odie. A green alien finds the dog, scans its brain and decides to help him bu switches his mind with its orange tormentor's. Garfield now experiences Odie's dog life and soon experiences the dangers of trying out canine fun. Worse, the dumb pup is in danger of wrecking his new feline body by taking crazy risks. When the alien checks on them, disaster seems inevitable.
Bone Diggers: Unwilling to throw Odie even a second bone from the pork chops he steals from the fridge, Garfield sends Odie digging up one from the garden. When the dog drags in a giant one, Garfield realizes it must be from a Dinosaur and brings it to the city museum, bent on a huge finders' reward. Only there he discovers the home where it was found will be expropriated and torn down, so he flees with it. Then he trick the mean chihuahua guard dog next door and the archaeologists.
To Garfield's dismay, a fat cat claiming to be heroic secret agent X finds all other pets his captive audience and is served all treats, even Garfield's dinner. So his mouse friend is recruited to impersonate the fictitious foreign master spy who is supposedly out to kill agent X. Thus they find out he truth.
Garfield aways was friends with a resident mouse, but now it brought dozens of 'homeless relatives', who are willing to wait on the cat hand and foot but soon get discovered. So Jon demands their eviction or he'll hire an exterminator and pay for it with Garfied's lasagna budget. The cat can't physically pull that off, but thinks up an alternative.