Santanu, king of Hastinapura, was married to the beautiful Ganga, who was the river goddess in disguise. She agreed to marry him as long as he never questioned her actions. Over the years they had seven sons, but Ganga threw each one into the river. Santanu was distressed but he kept his promise. Finally, when their eighth son was born, Santanu asked his wife who she really was and why she had done this. Ganga revealed herself and told that her children had once been celestial beings, but were cursed to become human. She had ended their punishment quickly by drowning them immediately at birth. But since Santanu had questioned her actions, she left him, along with his last son Devarata.

Devarata is better known by his later name Bhishma. He receives this name, which means of terrible resolve, after vowing never to marry or have children. His father wanted to marry again (Satyavati, mother of Vyasa), but the conditions of the marriage were that the second wife would be the mother of a king someday. Honoring his father's wishes, Bhishma makes his vow, guaranteeing that neither he nor a son of his will challenge the claim to the throne.

Years later, one of Bhishmas half-brothers dies in battle, and the other becomes old enough to marry. On behalf of his half-brother, Bhishma abducts three sisters and fights off all their suitors. On returning home, he learns that one of the sisters, Amba, had already chosen a suitor. Bhishma allows her to leave, but her betrothed does not want her any more. Now abandoned, she returns to Bhishma and demands that he marry her. Ever faithful to his vow, Bhishma refuses. Amba then vows that one day she will kill him, even though the gods have granted Bhishma the power to choose the day of his death, because of his vow.

The importance and power of vows is evident throughout the epic. Once stated, a vow becomes the truth and must be fulfilled, no matter what else may happen. When his father and both his half-brothers die prematurely without children, Bhishma refuses to marry his step-brother's widows (Ambas sisters). He will not relinquish his vow, even though his celibacy makes no difference anymore.

The young princesses must be given children, but who can father them? There are no other men in the family besides Bhishma, and he has renounced women. So Satyavati, the king's second wife, asks her first-born son, Vyasa the poet, to give children to the two princesses. He goes to them, but the princesses dislike him, for as an ascetic who has taken a vow of poverty, he is filthy and smells. He explains to them that they will each bear a son: however, the first will be born blind because the first princess closed her eyes when seeing him, and the second will be pale-skinned because the second princess became pale at his touch. The blind son is called Dhritarashtra, the pale one is Pandu. Vyasa has a third son Vidura by a handmaiden.

As his brother is blind and unfit for the throne, Pandu becomes the new king of Hastinapura. One day while hunting in the forest, Pandu shoots a gazelle in the act of mating. The gazelle was actually a brahmin priest in disguise, who curses him saying that should Pandu make love to either of his two wives (Kunti and Madri), he will die instantly. Knowing he can never have children, Pandu resigns the throne, and goes to live with his wives in the mountains. Kunti, his first wife, informs him that she possesses a magic power. By reciting a secret formula, she can invoke a god at will and have a child by him. The mantra's power is put to the test, and three sons are born to her: Yudhishthira, the first-born, truthful and virtuous, son of the god Dharma; Bhima, the strongest of men, son of Vayu, god of the wind; and Arjuna, an irresistible warrior, son of Indra. Madri, Pandu's second wife, makes use of this power too. She gives birth to twin sons, Nakula and Sahadeva. Thanks to his two wives, Pandu now has five sons directly descended from the gods, the Pandavas, the heroes of the epic.

Years later, Pandu one day surrenders to his passion for Madri. Fearing for his life, Madri tries to push him away but her struggles only inflame his desire more. Once they make love, Pandu falls dead, fulfilling the curse, and Madri, devoted to him always, joins him on the funeral pyre.

Meanwhile, Dhritarashtra has become king, despite his blindness. He weds Gandhari in an arranged marriage. When she learns of her husband's infirmity, she decides to cover her eyes with a blindfold which she will never remove, to join him in his world of darkness. Then, after an abnormally long pregnancy of two years, she gives birth to a ball of flesh. Vyasa tells her to split up the ball into 100 parts and put them in jars of ghee (Indian butter); in this way she becomes the mother of one hundred sons, the Kauravas.