Overview: After Dennis and Walter mess up a science experiment, De Testa decides the boys would benefit from a bit of mutual understanding, and issues the rivals with a formidable challenge - they must each live the other's life for a day... and if they fail the experiment, they'll be teamed up for everything, forever! Dennis must teach Walter how to menace - and learn how to behave himself - in record time, in order to avoid a fate worse than detention.