Emma Blackwell used to love mermaids. Jonah Blackwell used to love pirates. And William Blackwell tried to be a good captain. Which he would be, if he could get his twin brother and sister to stop fighting long enough to sail their boat, the Peregrine, across the bay. But after the Blackwells see a phantom ship, barely survive a terrible storm, and then mysteriously wake up with seaweed in their mouths, everything changes. They're becalmed in fog. They run aground on a strange island. They hear distant drums, and their weird adventure begins! The Blackwells face zombie pirates, terrifying mermaids, and a shipwrecked group of cursed ship's figureheads, including a Roman gladiator and an English knight, all led by the strange dolphin-boy, Finn.