The sage Ugrashrava known as Souti narrates the entire story of the Mahabharata to the sages in the Naimisha forest during the sacrifice being conducted by Shaunaka and other sages.
The story starts with the Sarpa or snake sacrifice being conducted by the King Janmejaya. The sage Vaishampayana, a disciple of Vyasa narrates the story of his ancestors to King Janmejaya.
The demons or the Asuras take birth on earth in various forms and they cause trouble on earth by harassing humans. The Gods then go to Brahma, the creator who tells them to take birth on earth. He tells them that Lord Vishnu, the preserver will be born on earth to lead the fight against evil so the burden on earth can be reduced.
The main story starts with King Shantanu of Hastinapura who marries Ganga who has to come to earth due to a curse. Ganga puts a condition that Shantanu should not question her to which the king agrees. Ganga has a son from Shantanu but drowns the child in the river. A shocked Shantanu keeps quiet, but this repeats seven times.
When Shantanu stops Ganga while she drowns the eighth son, she tells him that his sons are the Vasus who were celestial beings under a curse. They were liberated when she drowned them. She then takes the eighth son named as Devavratha and he is trained under the great warrior brahmana Parashurama.
She then hands over Devavratha to the king and leaves to her world. Devavratha is named the heir to the throne. One day Shantanu sees a fisherman’s daughter named Satyavathi and falls in love with her. Her father puts a condition that he can marry her only if their son is made the next king.
To ensure his father’s desire is fulfilled, Devavrata takes a vow of being a brahmachari or celibacy for his entire life and promises to serve Hastinapura. For this terrible (Bheeshma) vow, he becomes Bheeshma.
Shantanu has two children Chitrangada and Vichitravirya before he dies. Chitrangada becomes king but his reign is cut short when a Gandharva by the same name kills him in the battle. The young Vichitravirya becomes king with Bheeshma assuming the role of regent.
Satyavathi is anxious to continue the lineage of the Kuru kingdom of Hastinapura. Bheeshma learns that the king of Kashi has arranged a Swayamwara for his three daughters Amba, Ambika, and Ambalika. The king has not sent an invitation to Hastinapura. An enraged Bheeshma defeats the assembled kings and brings the three princesses to Hastinapura.
One of them Amba wants to marry Shalva and hence requests Bheeshma to release her. She is released but Shalva refuses to marry her. She then asks Bheeshma to marry her but he refuses. She later dies after taking a vow that they she will be reborn to kill Bheeshma in her next life.
Ambika and Ambalika are married to Vichitravirya. Unfortunately, the king suffered from consumption and died after seven years before producing an heir. Satyavati then summons Bheeshma and asks her to give up his vow, marry the queens, and rule the kingdom.