Matsya is the avatar of Lord Vishnu in the form of a fish. Often listed as the first avatar in the list of the ten primary avatars of Vishnu, Matsya is described to have rescued the first man, Manu, from a great deluge. Matsya may be depicted as a giant fish, or anthropomorphically with a human torso connected to the rear half of a fish.
Matsya Avatara is to show how lord Vishnu incarnated himself on this earth as a fish only to save the universe from the demon Hayagriva and returned the Vedas to Brahma. There once lived a king by name Satyavrata who was an ardent devotee of Lord Sri Vishnu who gave up his kingdom to his future generation and performed penance by living only on water. One day as he was getting ready for his penance and having his holy dip in the water a tiny fish came out of his kamandalu on to the palm of his hand. The king who was soft-hearted without causing any damage to the fish let it go free in the river. But he was surprised to see the fish talking and asked him not to let it in the river for there are big creatures to eat it up. Feeling pity on the fish Satyavrata put it back into his kamandalu.
The next day Satyavrata was surprised to see that fish has grown and was finding difficult in the kamandalu. So he transferred it into a bigger vessel. Then again he found it growing bigger and bigger day by day and then he had to transfer it into a lake. One day he was surprised to see the fish huge and enormous and it could find itself comfortable only in an ocean. Here too, the fish was not happy, for it was scared that alligators and other animals of the water would kill it. The king was confused and only then it struck Satyavrata that the fish was none other than Lord Vishnu who has incarnated as a fish for some specific purpose. Sathyavratha prostrated before the fish with folded hands and said that he is convinced for he realized that the fish was none other than Sri Narayana. He also told that Lord Narayana comes down to the earth when the need arises and to save the people and to enhance the welfare of his people.
Now the Lord answered to him saying that on the seventh day from that day, the earth will be uncontrollably drowned in the waters of pralaya and at that time a boat will come to rescue him. But before that Sathyavrata has to go and collect the herbs, plants, seeds and tress and all the different species of animals along with seven rishis. And when the storm hits he has to get into the boat along with all the things and the rishis. When the storm really gets very bad Sathyavratha has to tie the boat with Vasuki, the serpent, to the Lords horn. Sri Naryana also warned for it will be very dark everywhere but still the lights from the rishis will illuminate Sathyavrata and the surroundings. The Lord also told that he will support him during the time of pralaya after which Sathyvratha shall gain supreme wisdom from the almighty. Sathyavrata followed all the instructions of Sri Narayana and did as he was told. Exactly on the seventh day the storm and the flood started. The storm and the flood were so severe that there was water everywhere. The boundaries of the earth were all lost. The good king and the seven sages sat in the boat in deep meditation worshipping the almighty to shed all the ignorance of the people when they worship Sri Narayana and Satyavrata listened enraptured, as the divine Lord taught him the true nature of the self.