This happened a very, very long time ago. There was a kind king named Satyavrat. One morning, he was offering water to his ancestors on the bank of the river Kritamala. When he cupped water in his hands, he saw a tiny fish swimming in it.
The king was about to put it back in the river when the fish spoke: "O King, please don't leave me in the river. The big fish here will eat me. Please save me." The king's heart filled with kindness. He put the fish in his kamandal, his water pot, and took it home.
The next morning, the king saw that the fish had grown so big that it filled the whole kamandal. The fish said, "O King, there is no room for me here. Please give me a bigger place." The king put it into a big well. In a short while, the fish filled the whole well! Then the king put it into a large lake. The fish grew so much that even the lake became too small.
At last, the king carried the fish to the ocean. He folded his hands and said, "No fish grows like this. Who are you? You must be Bhagwan Vishnu himself."
The fish smiled and said, "Yes, O King, I am Vishnu. On the seventh day from today, a great flood will come, and the whole world will be covered with water. Then a big boat will come to you. Put seeds of all plants and healing herbs in it, take creatures of every kind with you, and seat the seven great rishis in it too. I will come. Then use the great serpent Vasuki as a rope and tie the boat to my horn."
On the seventh day, heavy rain really did fall, and the ocean began to rise. Just then a big boat arrived. The king did exactly as Bhagwan had told him. Then a huge golden fish with a single horn on its head appeared in the water. The king tied the boat to its horn with Vasuki.
For a long time, Matsya Bhagwan pulled the boat safely through the waves. On the way, he taught the king and the rishis about the soul and about God.
Around that time, an asura had stolen the Vedas from Brahma ji while he was asleep. Matsya Bhagwan defeated that asura, took back the Vedas and returned them to Brahma ji.
When the waters went down, the world began again. King Satyavrat later became known as Vaivasvat Manu. It is from Manu that we humans get the name "manav".
The lesson Be kind to even the smallest creature.
Source: Bhagavat Purana, Skandh 8, Adhyay 24 (retold); the story is also told in the Matsya Purana
