While playing on the beach, little Sosuke finds a small red fish trapped in a candy jar. The boy frees the fish, whom he names Ponyo, and promises to protect her forever. However, Ponyo is forced to return to the depths of the sea by her father, a powerful sorcerer. Determined to reunite with her new friend, Ponyo decides to become human. In her attempt to escape the sea, she accidentally spreads a magical potion through the water, transforming her sisters into giant waves that threaten to flood Sosuke’s village.