Children's comedy about a grandpa who can magically shrink. When Grandad Godfrey comes to stay, Great Aunt Loretta mixes up the two grandfathers' caps.
When Grandad Godfrey comes to stay at the Mill on the Marsh with his grandson, it turns out that they are huge Captain Dumbletwit fans, just like Josh and Grandpa. What is more, both grandfathers own identical black caps, with one exception - only Grandpa's cap is magic.
Great Aunt Loretta mixes the caps up, so when Grandpa needs to shrink to rescue the suitcase key that's fallen down the mousehole, the cap doesn't work. Josh has to swap the caps back before their guests find out. However, they're about to leave as they are not happy with Great Aunt Loretta's bizarre hospitality.
Josh finally succeeds in retrieving the magic cap. Grandpa quickly shrinks and finds the lost key so that Grandad Godfrey can open his suitcase. Much happier, the guests decide to stay. Grandpa comes back to his normal size and soon they all playing Captain Dumbletwit game together.