A dolphin comes up the channel and hangs around the Wedloe's dock. He and Ben become friends and Mark considers him his pet. Mark is disappointed when he finds out that the dolphin escaped from Miami Seaquarium and has to go back before he is lured by the wild dolphins at sea in mating season. The lure of mating season is too strong for the dolphin, Marco Polo, and he heads to sea with Ben following.

Mark is despondent as he has not just lost his new pet, Marco Polo, but his old pet Ben. Frank from Seaquarium takes Mark and Tom out on a boat to find Ben and Marco Polo. They spot Ben on a small island and as they get his attention and he swims for the boat he is threatened by a shark. Marco Polo comes to the rescue and nudges the shark away from Ben. Ben climbs a ladder to get onto the boat, and Marco Polo is returned to Seaquarium.

After Mark has a series of mishaps that soil three pairs of Tom's pants, Tom is angry at him. Feeling unwanted, Mark moves in to Ben's shed. While fishing, a fisherman comes by the river and hires Mark as his guide. The small boat overturns at a rough spot in the river and Mark and Mr. Carter are threatened by alligators and a cougar. Tom hears the commotion and scares off the alligators while Ben scares off the cougar. Mark and Tom have a heart-to-heart conversation and clear the air between them.