Joseph, the daughter of Captain Corcoran of the British ship HMS Pinafore, is in love with Ralph Rackstraw, a lower-class sailor, but her father intends to marry her off to the well-bred Sir Joseph, First Lord of the Admiralty. Meanwhile, the captain’s former nanny, Little Buttercup (now a bumboat woman), is in love with him, but because of his high rank, he hesitates to reciprocate. Josephine and Ralph plan to elope, but their plan is foiled by Dick Deadeye, an unpleasant member of the crew, who tell the captain. However, when Sir Joseph has Ralph imprisoned in the brig, Little Buttercup reveals to everyone a secret she’s been hiding for years, which dramatically changes everything in the end.
