Dennis and his faithful hound Gnasher are out to make every day as much fun as possible, but their adventures often lead them into trouble. Dennis decides to get a pet pal for Gnasher to keep him company while he's at school. After a few misfires, they find the perfect playmate: The Abyssinian Wire-Finned Piranha. However, they find out too late that there is only one condition - they must never, under any circumstances, feed it pies... Uh-oh.