Animation. Dennis lives in Beanotown with Mum, Dad, little sister Bea and his pet dog Gnasher. Gnasher is more than a dog to Dennis; he is a kindred spirit. Whatever Dennis does, Gnasher does with him. Together with their friends Curly and Pie Face, Dennis and Gnasher believe the world truly is their oyster and that kids should make the rules. Not everyone agrees, including Mum and Dad. But Dennis and Gnasher are not going to let that stand in the way of their pursuit of fun! Tired of Dennis's childish antics, Dad is determined to teach his son how to behave more like a grown-up. Dennis is forced to accompany Dad to his school reunion party. Dad is looking forward to reminiscing about the old days with his old school chums, dressed in traditional school uniform as per the invitation's instructions. Upon arrival at the party, it is revealed that Dad has been tricked by Walter's dad - all the other guests are in formal party wear. Outraged that Dad has been humiliated, Dennis urges him to take action. Father and son join forces in their revenge and Dennis sees another side to Dad - a prankster side! A barrage of water balloons later, Dad is ready to call things even and get back to the party, but Dennis will not let it end so easily. Using Dad's old menacing inventions, Dennis inadvertently gets Dad into trouble with Mrs Creecher. There is only one way to clear Dad's name - by showing everyone who the real menace of Beanotown School is.