One day there was a man named Toba who was looking for fish in the river. Once, the young man's hook was caught in a goldfish, which suddenly turned into a beautiful woman. The young man also took the woman's wife. With the condition, he may not reveal the origin of his wife to anyone.

They both live happily and are blessed with a child named Samosir. One day Samosir was given the task of bringing lunch for his father in the fields. However, Samosir forgot and instead played with his friend. Due to hunger, Toba was angry and accidentally said "You fish!" Immediately the earth shook. Water popped up from the cracked ground. It was raining heavily.

Toba realized and regretted that he had broken his promise to keep his wife secret about the origins of his wife. However, it was too late. The disappointed mother returned to being a fish. The rain that didn't stop gave rise to a large lake called Tao Toba. Samosir the child, because of his mother's instructions, survived by climbing a high hill in the middle of the lake, which became known as Samosir Island