The day has come for the family walk. Perfect Peter is all happy and can't wait to wear his new books. Horrid Henry wants to do anything but this. His parents think he spends way too much time indoors. Worse yet, they're making him go to the country where he'll probably get eaten by goats. They walk and they walk and they smell the rich smell of manure, "Peter, couldn't you wait?" How can he get himself out of this?

For fun, Henry decides to pull the greatest trick ever on Perfect Peter. With a few turns of the dials and clicks of the switches, Henry can go anywhere he wants in his time machine. To the past for great adventures? No, why not try the future? And why not trick his brother while he's at it?