One day Lord Brahma approached Lord Vishnu, and said that He had foreseen a disaster that would occur and wanted to warn Vishnu. Brahma told Vishnu that it was his responsibility to protect people.
One day Brahma was in a deep sleep. He kept with him the Vedas. These are really important books, like the blueprints for the whole universe. While he was asleep, a demon named Hayagriva stole them. Hayagriva swallowed the four Vedas, and hid deep inside the ocean. Without the Vedas, the world was in a lot of danger of going completely dark, like the lights going off when there is no electricity. To protect the world, Lord Vishnu incarnated as Matsya to save all that was good in the world before its destruction.
At that time there was a pious king called Satyavrata who was a great devotee of Lord Vishnu. Satyavrata was down by the river for his morning prayers to the god Vishnu. His morning prayers were his daily ritual to lord Vishnu. After he picked himself up off the grass, Satyavrata went over to the river to wash his hands and cleanse himself. While he was washing his hands a tiny fish swam into the gap of his cupped hands. The fish was extremely small, no bigger than his pinkie, and he assumed it was just a baby. The fish appeared frightened. Satyavrata spoke the language of the animals and proceeded to try to comfort the small fish so that it did not die from shock. The fish was pleading for his life but Satyavrata told him there was no need because he could never harm such an innocent creature. Once Satyavrata began to talk, the fish immediately calmed down. Satyavrata found out that the name of the little fish was Matsya and that he had been separated from his large family by getting caught in a strong current.
Satyavrata felt bad for Matsya, and decided that he was going to take care of him. He cupped Matsya in his hand with plenty of water and walked his many miles home. He was extremely careful the whole trip and never let any of the water leave his hand. When he reached home, Satyavrata showed his wife the small little fish and asked her to find something to keep him safe. His wife, Neha, brought him a small little cup, which was more than enough room for Matsya. Matsya knew that he would always be safe in Satyavrata and Neha's protection and so he swam around in joy.
Matsya was not a normal fish, though, as they would soon find out. Within days, Matsya had grown to the size of Neha's small hand. She moved Matsya to a large bowl to make sure that he always had enough room to swim. A few days later and Matsya had already outgrown the bowl! Satyavrata carried the fish to the small pond that was behind their home. This growing kept happening until Matsya had outgrown the pond, a stream, a lake, and a river. Satyavrata was not going to give up on Matsya because he felt an unnatural devotion to the once small creature. He was extremely drawn to him and felt that he must protect him at any cost. Matsya, in return, was always grateful for the help of Satyavrata.
The next move and presumably the last was Satyavrata putting Matsya into the ocean. He was in doubt that this would even be big enough to hold him but Satyavrata was out of options, so he got on his knees and prayed to Vishnu. This triggered something in Matsya. He revealed himself as Vishnu and he began to speak prophecy. He told Satyavrata that he had one week to prepare for a flood that would cover the entire earth, destroying everything. He instructed Satyavrata to take all medicinal herbs, all the varieties of seeds, and to bring the seven saints along with the serpent Vasuki and other animals, and to build a big boat to put them all in. He told Satyavrata that he would be back at the end of those seven days before the flood begins.
Then to restore the Vedas, Matsya dived into the ocean to kill Hayagriva. A furious battle ensued between Lord Vishnu and demon Hayagriva for seven days, in which Hayagriva was defeated and killed. Vishnu restored the Vedas to Brahma. Because there was so much splashing and fighting, a big deluge (rainfall) occurred all over the Earth. Everything was in a big flood!
Satyavrata made all arrangements as Vishnu had advised him and at the end of the seven days, Vishnu appeared. The lord advised Satyavrata and others to board the boat and fastens the serpent Vasuki to his horn as a rope to the boat, and sailed all of them to a safe place.
Matsya told Satyavrata that he had been chosen because he was the most kind and loving human Vishnu had ever seen. Satyavrata had shown his devotion to not only a god but also to a small helpless creature. Vishnu said there was hope in the world if all men were to show so much love to all creatures.