Sungai Brantas tercernar. Airnya bau dan banyak sampah. Ada pabrik kertas didirikan di pinggir sungai. Pabrik itu nnembuang limbah tanpa diolah. Prigi dan ayahnya ingin berbuat sesuatu agar Sunsai Brantas bersih kembali. Mereka menemui banyak rintangan, dan perjuansan yang panjang. Bagaimana cara Prigi dan ayahnya menjadi "Icesatria Sungai"? The Brantas River was polluted. The water smelled bad, and trash floated everywhere. A paper factory dumped its waste into the river. Prigi wanted to make the Brantas River clean again. But that would require a very long fight. How could Prigi make the river clean and become "The River Warrior"?