On a family trip to the seaside, Shirley’s parents relax in deckchairs—only bedlam ensues as Shirley embarks on her own grand adventure. With her imagination leading the way, she turns the rocky shore into a pirate ship, sails the high seas, and hunts for buried treasure. All this unfolds despite her mother’s repeated cautions about hazards like tar, cold water, and stray dogs. Through witty alternate illustrations, the story humorously contrasts Shirley’s bold exploration with her parents' sleepy daydreams.