Sixteen-year-old Ramun Salazar longs to become a pearl diver. With his father out of town, Ramun dives in a remote lagoon and finds a huge black pearl, the Pearl of Heaven, in a forbidden cave. According to Indian legend, the cave is guarded by the Manta Diablo, a demon incarnated as a giant manta ray. Ramun's father makes an offering of the pearl to the Virgin Mary for the safety of the pearl fleet. On the next voyage, the fleet is destroyed by an unexpected storm. Ramun believes that the Manta Diablo is taking revenge for the loss of the Pearl of Heaven, and he is determined to return the pearl to its undersea cave. An arrogant, unscrupulous pearl diver, Gaspar Ruiz, steals the pearl and forces Ramun to row across the sea with him. When the Manta Diablo attacks the two, Ramun is certain the...