The Underdogs are overdue for a holiday! So when the Mayor of Cape Dog asks for help catching a mysterious sea monster that is terrifying the citizens – the Underdogs head for the coast! Will they be able to catch this mystery monster? Will they find the missing Top Dogs? And will Carl get to go on the Splashinator ride at the brand-new Wet Dog Waterpark? Join the crew as the Underdogs set sail on ruff seas in their fishiest mystery ever!