Ponyo is a sweet-faced goldfish from a magical realm under the sea. One day, Ponyo's curiosity takes her far from home. After accidentally hitching a ride on a jellyfish, Ponyo washes up on the shore and is found by Sosuke, a kindly human boy. The two become fast friends and are swept away on an incredible adventure.

A fierce storm rages, and Sosuke and his mother drive against the wind. Suddenly Ponyo appears, running along the tall waves. At once fish and human, when reunited with Sosuke, Ponyo becomes a beautiful red-haired girl. The three weather the storm at Sosuke's house, while out on the ocean both Ponyo's and Sosuke's father encounter a vision they won't soon forget.