Children's comedy about a grandpa who can magically shrink. Mr Mentor the inventor has a bright new pupil, Billy Bentice.
Mr Mentor the inventor has a bright new pupil, Billy Bentice, who arrives at the lighthouse, eager to learn from his hero. When Grandpa hears about this, he reaches straight for his shrinking cap. Mr Mentor's inventions never work without Grandpa's help! Sure enough, the Wizzywoodlesome Waker Upper is malfunctioning, so Grandpa jumps inside to fix it.
But Billy wants to investigate the problem for himself - he blocks up the exit to the machine and Grandpa gets stuck inside. Then Billy decides to invent something for himself and creates a Tippy Toe Tell Me which can identify people's footsteps. Meanwhile, Josh has been worrying about Grandpa so he gets Auntie Jules to drop him off at the lighthouse. Billy demonstrates his invention which declares that Grandpa is in the room. Mr Mentor thinks this means it's not working, but Josh knows better.
He eventually works out where Grandpa is and rescues him. And Mr Mentor has to admit that Billy Bentice is an excellent apprentice.