One of the king's ministers was a man named Manu. While Manu was washing in a river, a little fish swam into his hands and asked him to save his life. Manu rescued the fish by placing it in a jar but the fish grew too large for the jar. Manu moved the fish to a tank but the tank was soon too small as well. Manu then moved the fish to a river and when that was not large enough, he moved the fish to the ocean.
This fish, which Manu saved so many times was actually an incarnation of the Lord Vishnu. Vishnu warned Manu of a great deluge which would soon cover the earth. So Manu build a large boat and took his family and the seeds of life on board. When the flood waters came, the divine fish towed the boat to a mountaintop and there it rested until the flood waters receded.
Scene from a Matsya Purana: Manu with Seven Rishis in a boat tied by Vasuki to Vishnu; Indra and Brahma show deference to Vishnu as Matsya avatara. Mewar, circa 1840
Incarnation of Vishnu as a Fish, illustration from a devotional text. Artist unknown. nd.