On his eleventh birthday, schoolboy Magnus Fin found out that he was half selkie -- part human, part seal. Although he looks like a boy and lives on land, he can breathe underwater. When Magnus Fin discovers his initials scratched into the rocks by the shore and finds dead seals washed up on the beach, he knows his selkie family needs his help, and he dives down beneath the waves to find out more. The great seal sickness has struck and his grandmother Miranda is dangerously ill. But Magnus Fin is sure there's another reason for their affliction. He sets out to reveal the truth, with just his moonstone, his last baby tooth and some seaweed from Neptune's garden for luck. Little does he know that his best friend Tarkin, who can't swim, is determined to join him on this perilous mission.