The robots have built a racecourse for their toy boats. Oh. One toy boat's gone too quickly and has leapt out of that water, bumping into some rocks. The boat's propeller is broken. They'll have to stop the race and go to Lifty's shop. They couldn't find the propeller they needed there. The robots are very upset. Leo the Truck has a great idea: they should build a 3D printer and print off the broken part.