Thomas loves his summer visits to his grandmother's on Cape Cod. He spends his days wondering about the sailing ships of the past and imagining their stories.

One afternoon, after a night of terrible thunderstorms, Thomas finds, deep in the sands, a weathered old-fashioned belt buckle. When he picks it up, a boy his own age appears before him. His name is Silas Rich, a cabin boy from a ship called the Monomoy that sailed almost 250 years ago. As Silas tells his tale, suddenly the world of sailing ships is very near indeed.