Matsya - The Fish
Lord Vishnu first reincarnation was a
fish.
A king named Manu was performing
ablutions with river water when a
small fish jumped into his hands.
Just as the King was going to throw
the fish back into the river, the fish
begged the king to save him from the
other big fishes within the river.
The king listened to the small fish
and placed in him in a small jar.
Slowly, the fish began to grow and
soon the King had to throw it in to
the river, and then in the Ganges and then in the ocean.
The King realized that the fish was the Supreme Being and bowed down to the fish. Lord Vishnu made a apperance and asked the Kking to build a huge boat to take the seven sages, seeds and one animal of his type on board due to the coming of a flood. Lord Vishnu said that the fish would then come back and propel the boat to Mount Himavan where they would survive the flood.
The King built the boat, boarded the animals and waited for the flood to come. Lord Vishnu appeared as a fish, the King tied the boat to the fish and Lord Vishnu took them all to Mount Himavan. The king started procreation for the new era.
The video below, titled 'Dashavtar Illustrated Story - Matsya Avatar ... The Fish Incarnation', is a great representation of the story.