Young Frederic is mistakenly apprenticed to pirates until his twenty first birthday. Helping Frederic to cope is the brash Pirate King, Ruth the pirate maid, romantic Mabel, and the delightfully stuffy Major-General Stanley.

On the rocky coast of Cornwall, pirates celebrate the coming of age of their apprentice, Frederic. Although Frederic's apprenticing was the mistake of his nurse maid Ruth, he has dutifully served. Now he announces he will fight against piracy.

Frederic enlists a group of beautiful girls and their father, the delightfully dotty Major-General, and some bumbling policemen. But Ruth and the Pirate King tell Frederic that his apprentice papers state that he won't be of age until his 21st birthday, which won't occur until the distant date of 1940 because Frederic was born in leap year on the 29th of February. Frederic is doomed to remain the pirate apprentice. The policemen try to capture the pirates on their own, but are easily defeated.

Everyone is happy to discover that the pirates are really all "noblemen who have gone wrong", so all is forgiven in the end.