According to Hindu mythology, there are four yugas – Satya-yuga, Treta yuga, Dwapara yuga and Kali yuga. Brahma is the creator who does all creation and he gets power for creation from Vedas.
Lord Vishnu is the God of Preservation, to save Brahma’s creation from the evil he takes different forms and ends the evil.
Lord Vishnu took ten different forms called as Dashavataras.
Let’s look at these incarnations their need and characters involved in them.
The first avatar of Lord Vishnu is called as Matsya Avatar – “Fish”.
At the end of Satya Yuga, there would be a great flood which will destroy all the life on earth and life would start fresh again in next yuga that is Tretha. Lord Brahma sleeps between the gap of yugas creation. During his sleep, a demon named “Hayagriva” emerged from the nose of Brahma and knowing Brahma is in deep sleep stole the Vedas with the intention to gain all the knowledge and dived deep into the ocean and hid down there.
Lord Vishnu saw all this and to stop asuras knowing the knowledge of Vedas, he decided to take the form of fish “Matsya” and get back the Vedas.
In this avatar lord, Vishnu had an obligation to fulfill King Manu’s only desire to see Lord Vishnu with his eyes in his lifetime once for which he performed penance for thousands of years. To fulfill King Manu’s wish Vishnu took the form of a small fish and when Manu came to the river to offer prayer early in morning appeared in front of him and asked him to protect him from other big fishes in the river. Being the king Manu took fish in hand and put him in a small jar and took him to palace, within few hours the fish grew larger and larger and Manu has to take fish to the ocean for its size, the fish had a horn on its head. On reaching ocean Manu understood that it was none other than lord maha Vishnu himself and bowed in front of him.
Lord Vishnu told Manu that a flood is coming which will wipe out all the life on earth so build a big ship and take a pair of all the life forms and trees with him in it so that after the flood he can start the life again on earth with them.
Saying so lord Vishnu dived into deep ocean where Hayagriva was hiding and after a fight maha Vishnu stuck Hayagriva with his horn and killed him.
Manu completed building the ship and collected all animals as instructed by the lord, rains started and soon there was a huge flood Maha Vishnu appeared again and asked Manu to tie the ship to his horn with Vasuki as rope and dragged the ship into safe waters and kept it there till the floods calmed down.
Lord Vishnu in Matsya avatar taught the Vedas to Manu and the others during the flood and put the ship at Mount Himavan for the people there to continue the new yuga.