Could there be a pirate treasure buried in Bayport? Captain Sid, the owner of a souvenir shop, says there is. When Frank, Joe, and Chet meet Sid on their way to go sailing, he tells them about a pirate ship that sank off the coast of Bayport four hundred years ago.

As the boys head to the boat, they find a strange silver coin on the ground...then an old bottle with a treasure map inside...then a rusty musket ball—the kind pirates used in battle! Could Captain Sid have been right about the shipwreck? Frank and Joe are baffled, and it won't be smooth sailing until they figure out this whale of a mystery