海神的女儿波妞从深海出逃玩耍，却被困在玻璃瓶里冲到岸上，碰巧被住在岸边悬崖的宗介救出，两人因此相识。但是波妞的父亲——魔法师藤本，认为人类世界肮脏丑陋，强行把波妞带回海里。面对父亲的阻挠、席卷小镇的海啸，以及永远失去魔法的威胁，波妞仍然大胆而坚决地决定——要变成人类跟宗介在一起。而承诺要永远保护波妞的宗介，也即将面临新的挑战...The son of a sailor, 5-year old Sosuke lives a quiet life on an oceanside cliff with his mother Lisa. One fateful day, he finds a beautiful goldfish trapped in a bottle on the beach and upon rescuing her, names her Ponyo. But she is no ordinary goldfish. The daughter of a masterful wizard and a sea goddess, Ponyo uses her father's magic to transform herself into a young girl and quickly falls in love with Sosuke, but the use of such powerful sorcery causes a dangerous imbalance in the world. As the moon steadily draws nearer to the earth and Ponyo's father sends the ocean's mighty waves to find his daughter, the two children embark on an adventure of a lifetime to save the world and fulfill Ponyo's dreams of becoming human.