Batara Guru, a fisherman, catches a talking fish who begs to be released. He lets the fish go, and she transforms into a beautiful woman. They fall in love and marry, but Batara Guru promises never to reveal that his wife was once a fish. However, during an argument with his son, he breaks this promise. Upon learning her secret, his wife becomes angry and causes volcanoes to erupt and form a large lake now known as Toba Lake.