In 2500 AD, the glaciers at the two poles of the earth melted in large quantities, and the earth became a vast ocean. People could only survive on the water, and built floating islands on the water. Soil became a rare commodity. One day, a lonely sea walker (Kevin Costner) came and exchanged a can of soil for fresh water and tomato seedlings. Just as he was about to leave, the locals discovered that he had gills and webbed feet, and thought he was a monster. They locked him up and planned to execute him in two days. Someday, pirates attacked the floating island. Helen (Jeanne Tripplehorn) and her adopted daughter Ilona (Tina Majorino) failed to catch the escaping lifeboat, and they rescued the sea walker. With his extraordinary wit and courage, the sea walker escaped with Helen and Imona. It turned out that the pirates' target was only Ilona, and Imona's life experience hid an amazing secret...