There was a young man named Toba who lived alone in North Sumatra. He was a farmer and sometimes fished in the river near his house to sell at the market. One day, he was fishing in the river and not long after, his fishing hook was eaten by a fish. Sure enough, Toba caught a large golden fish and took the fish home.

When he got home, suddenly the fish he brought turned into a beautiful woman and said, "Sir, I am someone who has been cursed by God for breaking the rules. I will turn into a human when someone touches me. Please sir, give me a place to stay.”

Toba, who was still stunned, agreed to the beautiful woman's request for help. They got to know each other and finally Toba fell in love with her. Toba married this beautiful woman and she officially became Toba's wife. They spent their days happily. One day, the wife became pregnant. She told Toba, her husband, about her pregnancy.

His wife told him that no matter what happened he must not call the boy “fish child”. Toba agreed to these terms. The child was finally born. It turned out that his wife gave birth to a boy. Toba was very happy with the birth of his first child and gave his child the name Samosir.

Samosir grew up to be a healthy but lazy child. He often refused when his mother asked him to deliver food to his father’s field. One day, Samosir was forced to deliver lunch. Feeling lazy, he walked reluctantly. On the way, Samosir felt hungry and decided to eat some of the food meant for his father, leaving only a little. When Toba opened the lunch box, he was shocked and angry to see just leftovers. Samosir admitted that he had eaten the food because he was hungry. Toba, who was tired and hungry, lost his temper and said,

"You little fish child!"

Samosir was shocked by his father's words and ran home crying. He told his mother, who felt sad and disappointed because Toba had broken his promise. Soon after, Samosir and his mother disappeared. The ground where Toba stood gushed out water, forming Lake Toba. The small island where Samosir and his mother vanished was named Samosir Island. It’s believed that a magical goldfish, the incarnation of Toba's wife, lives in the lake.