Garfield, Odie, and Nermal use their imaginations to enjoy a bath.
As Nermal walks through the neighborhood, admiring himself in a mirror, he finds a sign for a supposed contest, which leads him into a trap that launches him into a mud pile. Garfield and Odie, satisfied that the trick worked, inadvertently wind up in the same trap. When Jon finds them, he takes them inside to give them a bath.
Jon takes a break when the phone rings, so he leaves the trio to finish up bathing. When Nermal says that baths are boring, Garfield shows him otherwise by making up a story. The three pets swim down to the bottom of the tub, which they imagine as the ocean (they are able to breathe and talk underwater since Garfield is able to “make up the laws of science”).
The trio travel to the surface via bubbles; after their bubbles pop, they encounter a giant toy boat, as well as a shark that chases them. After getting on the boat, they eventually find out the shark is a wind-up toy. They get him to leave by winding him up, with the promise of him leaving them alone. Later, the trio encounters winged loofahs. One such loofah acts as their guide as well, as means of transportation by pulling the boat.
After passing by a waterfall (from a faucet), they encounter a group of mermaids, which Loofah says can bring them back to their tub. The mermaids are willing to oblige, under the condition that the pets deal with a monstrous rubber duck. The duck soon appears and attacks the boat. Garfield manages to drive it away by having the mermaids sing in their lousy way. The mermaids tell Garfield the way back (and give them sea-shell necklaces).
The trio find a gigantic drain plug. Although they are unable to pull it out, the shark from before manages to do so. They go through the drain and back in the tub. Jon returns right afterward, finding the tub empty and the pets clean. He goes to get towels, while Garfield goes to check on the lasagna. Both Garfield and Odie are certain that their experience was just a fantasy, but Nermal points out to the latter that they still have the seashell necklaces.
When Loofah warns the trio about the mermaids' singing, Garfield figures that it is due to a legend that such singing would cause them to crash their ship on the rocks. This alludes to the Sirens from Greek mythology.