Abandoned by her mother in childhood, Shenxiu is haunted by recurring nightmares of a creature she calls "Nightmare." Amidst these troubling dreams, she sets sail with her family on a cruise ship. One tempestuous night, she encounters "Nightmare" on the ship's deck, eerily humming her mother's lullaby and beckoning her towards the ocean's depths. In a fateful slip, Shenxiu tumbles into the "Deep Sea" restaurant below—a former submarine transformed into an underwater dining experience. It is here she meets Nanhe, the Chef, and together they begin an enigmatic voyage of discovery.