Matsya avatar was the first incarnation of Lord Vishnu in form of a fish. Raja Manu was the first human being on Earth and the son of Brahma. Shatrupa was his wife. One day, he found a fish in a river. He put the fish in a pot of water but it grew larger, then he put it in a pond but again it grew larger then he put the fish in a river but it outgrew again. Manu realised that the fish is a divine being and reveals itself as Vishnu. Vishnu warns Manu of a destructive flood and instructs him to make a huge boat which can fit plants, seeds, living creatures and the seven sages.
Manu does the instructed and carries everyone on the boat but the flood starts destroying the Earth. Then Vishnu appears as Matsya, and Manu ties the ark of the boat with Vasuki snake to the Matsya. Vishnu saves everyone on the boat and a new cycle starts guided by the restored Vedas. The Noah story mentioned in the Bible is a modification and copy of Matsya story while Adam and Eve are copied from Manu and Shatrupa.