Upon the death of his beloved Uncle Calvin, a young Jonah Padgett makes his way to Sag Harbor with nothing but a letter of introduction and a strange amulet shaped like a dolphin. He signs on to the whaler Absalom, under the command of his uncle's old friend, Captain Solomon. Upon rounding the Horn, the crew hears stories of a sperm whale said to be as big as a mountain with skin as black as a bible, and in command of a harem of a hundred females. Captain Solomon wagers that his crew will be the one to make its fortune hunting down the elusive "King Jim". But when the Absalom signs on the native called Koro as a harpooner, the ship is beleaguered by inexplicable disappearances and horrific death, which pits shipmate against shipmate. While trying to clear his friend of suspicion, Jonah finds himself adrift in a fantastical world filled with dolphin-like merfolk and nightmarish were-sharks, endangered equally by inhuman creatures from the ocean's depths and human fear and superstition, his very life at the mercy of something as old and mysterious as the sea itself.