The Parallel Universe is an alternate universe where the roles of cats and humans are reversed. Here, Garfield works as a cartoonist, with Jon and Odie being his pets. Arlene, meanwhile, works as a veterinarian.
The regular Garfield winds up in this universe when a wormhole opens up in front of the television set, which also brings the alternate Garfield into the former's world. In the end, both of them return, deciding that they are better off in their own worlds.