Twelve-year-old Merryn lives with her fisherman father in a little cottage by the sea. Each day, her father braves the tumultuous waves and returns home in time for dinner. One stormy evening, he doesn’t come back. Merryn has a vision that he’s been dragged underwater by a terrifying sea creature, and he needs her help. Determined to rescue him, Merryn builds a tiny submarine and embarks on a journey through the undersea worlds she’s only heard about in her father’s lullabies. As she faces the dangers and wonders of the world below the waves, she realizes that her father’s stories were all real.