Without going into an endless chain of cause and effects, the story of Mahabharata begins with King Shantanu of Hastnapur meeting Ganga (the river in her maiden form). He fell in love with her at first sight and asked her to marry him. She put forward a strange condition. No matter what she did, no matter how abhorrent her act, Shantanu would not question or stop her. If he did then Ganga would immediately leave Shantanu forever. The passions of the heart overcame the reasoning of the mind and Shantanu agreed. The two got married and soon Ganga was pregnant. As soon as the child, a son, was born Ganga took him to the river and drowned him. Shantanu, who was following her, was aghast but helpless. Six times again did Ganga conceive and she drowned six more newborn boys, making the tally to seven. But when this was to happen the eighth time Shantanu could not contain himself. He stopped Ganga and asked her what was she up to.
Ganga then recited the tale of the eight Vasus. They were the immortal sons of Manu. They had a friend Jitawati who was the daughter of Sage Ushinara. Since Jitawati was mortal the Vasus decided to do something to prolong her life. It was said that whoever drank the milk of the divine cow, Nandini, belonging to Sage Vashishta, would live for ten thousand years. The Vasus stole the cow but got caught. Sage Vashishta cursed them that they would have to spend one life as mortals on the earth. The Vasus then prevailed upon Ganga to become their mother on earth and kill them as soon as they were born so that they would fulfill the letter of the curse but circumvent its spirit. While Ganga agreed to kill the first seven, she insisted that the eighth, Dhyu, would have to live a complete human life. Else she would incur the wrath of the sage. "Events have proceeded as have been ordained," she said, "and this child you have saved is the eighth Vasu who will be known as Devavrata. For the time being I will take him with me but return him to you when he is sixteen." (This Devavrata, later known as Bhishma is considered by many to be the true hero of the Mahabharata.)
Shantanu was not satisfied. "Why did you choose me as your partner for this frightful task? Could you not find anyone else?" he said.