Sophie's grandpa loves making things. He spends hours in the workshop at the end of his garden, building all kinds of incredible machines. One day, a letter arrives-Grandpa is invited to a competition to find the world's greatest invention. He and Sophie try to decide what he should make-a mechanical octopus to explore the oceans? Or a giant rocket bus for tea parties on the moon? Eventually Grandpa comes up with an idea for the most amazing invention of all.