Short Synopsis (see below for a more complete synopsis):
In brief, there is a power struggle between two sets of cousins living in the palace of a king. One set of cousins (The Pandavas) is forced from the kingdom to live in exile for 13 years. When they come back to reclaim their place in the palace, the other group of cousins (The Kauravas) convince the king to wage war on the Pandavas. The battle rages for 18 days and in the end, The Pandavas successfully regain the crown, but all their cousins (The Kauravas) and all of their offspring are killed. And while the Pandavas survive, only ONE of their offspring (an unborn child) survives.
More Complete Synopsis:
In the Mahabharata, Pandu, father of the Pandavas, was crowned king of Hastinapur because his elder brother, Dhritrashtra was born blind and therefore unfit to rule. Pandu however was forced into exile after accidentally killing a sage while out on a hunt. In his absence, Dhritarashtra was appointed regent.
When Pandu died, his sons were brought to Hastinapur to be raised along with the sons of Dhritarashtra, the Kauravas. On coming of age, the Pandavas asked that the city of their father be restored to them.
Dhritarashtra, unwilling to give away what the Kauravas had come to believe was their birthright, decided instead to divide the kingdom. The Pandavas were given the wilderness of Khandavaprastha, on which they established the rich and prosperous city of Indraprastha. Because they ruled justly and upheld morals, the people of the their kingdom came to love the Panavas.
The Kauravas, envious of the Pandava fortune and still suspicious of them, invited the Pandavas to a game of dice, where the Pandavas foolishly gambled away the rights to their city for a period of thirteen years and were forced to live in exile.
During this time, the Mahabharata focuses on the travels through the wilderness of the Pandavas, as they listened to ancient religious stories and worked on both their religious practices and their martial skills.
After thirteen years of exile by the Pandavas, however, the Kauravas refused to part with the land. And so a battle was fought on the fields of Kurukshetra, raging for eighteen days, until the earth was soaked with the blood of a hundred thousand warriors.
At the crucial moment when the battle is about to start, the Pandava hero Arjuna loses his will to fight, realizing that he would be forced to kill his cousins. Krishna, serving as his charioteer, restores his confidence, not by justifying the war, but by explaining to him the significance of action in the cosmic plan.
Krishna's counsel to Arjuna is revered as, 'Bhagavad Gita,' the song of the lord. Krishna states that there is more to life than empirical reality. He says that the aging body and the wavering mind ensheath a divine essence - the serene soul. He explains how the immortal soul can be liberated from its mortal confines by realizing that all turmoil experienced by the body and the mind is a product of ignorance and attachment.
To facilitate this realization, Krishna offers the technique known as yoga, explaining its three different forms.
1) gyan yoga, logically becoming aware of the reality beyond perception
2) bhakti yoga, humbly accepting every situation as the will of the divine
3) karma yoga, fulfilling one's commitments without an eye on the results
Krishna explains that karma yoga is the best way to attain oneness with god, because it helps society function and it maintains worldly order, while also ensuring individual salvation.
After the war has ended and the Pandavas return to the palace, they take stock of all that has happened; they have killed off the entire lineage of their cousins and only one unborn child of their own lineage survived. The parents of the Kauravas leave the palace to live in the woods, but not before the mother curses the semi-divine Krishna to be killed by a mistaken hunter.
After the Pandavas rule the kingdom for many years, they decide to retire to the wilderness to devote themselves to spirituality. They embark on a pilgrimage to Mount Mehru, home of the gods, and along the way they slip and fall to their deaths, except for Yudhisthira, the leader of the Pandavas, who is known to be the most spiritually adept amongst them. Only a stray dog that follows Yudhisthira is all he has left for company.
Upon reaching the top of Mount Mehru, the God Indra rewards Yudhisthira by allowing him to enter the Kingdom of Heaven in his human form, but Indra will not let Yudhisthira enter while accompanied by the dog and Indra tells Yudhisthira to leave the dog behind. Yudhisthira responds that he would want know part of heaven that wouldn't allow as loyal a companion as the dog into it.
Indra reveals that this was only a test to see if Yudhisthira was truly ready to enter heaven, as the dog represents Dharma, and Indra wanted to see whether Yudhisthira would abandon Dharma in order to enter heaven.
Indra now allows Yudhisthira to enter heaven, but when he gets there, he is surprised to see that none of his cousins brothers are there, but instead his cousins, The Kauravas, are in heaven. When Yudhisthira asks to see his brothers, he is instead led to Hell where he is confronted with the terrible sounds of his brothers screaming in agony and the horrible stench of death.
Yudhisthira is given the opportunity to leave his brothers behind in Hell and return to Heaven if he so desires. But instead of forsaking the fellow Pandavas, he asks instead to stay in Hell with them so he may share in their suffering.
Indra reveals this was yet another loyalty test, and in reality The Pandavas are in Heaven and The Kauravas are the ones who are suffering in Hell. Yudhisthira then goes to join the rest of his Pandava brothers in Heaven.
Lessons and Contradictions from the Mahabharata:
The Mahabharata is filled with many plots and sub-plots and seems almost contradictory in the lessons it teaches. For example, in one section the heroes preach the value of telling the truth, yet lie to their enemies in battle. They agree to "rules of war" before the epic battle that they will not strike at the back or legs of a combatant, yet one of the main protagonists is incapacitated when struck in the thigh by one of the heroes.
And while one story after another teaches the importance of pity, compassion, and mercy, the heroes have little to no trouble slaughtering other humans in the epic battle. In fact, when one one of the heroes (Arjuna) starts to waiver in his determination to fight, feeling sympathy for the enemy combatants, he is lectured by Krishna to "fulfill his duty" to kill his enemies without being emotionally attached to the outcome.
One of the consistencies is the importance of parents, as both mother and father are venerated throughout the stories. Another consistency is the supernatural forces of curses and blessings having an impact on the lives of the characters. For example, it is a sage who curses the mother of when Dhritarashtra, telling her that her son will be born blind. That same sage curses the mother of Pandu, telling her that Pandu will be born feeble. When Pandu (father of the hero Pandavas) accidentally shoots a sage, as the sage is dying he curses Pandu, thus preventing Pandu from having children.
In fact, it seems that curses can be placed by either an affronted sage or by an emotionally distraught woman.
Conversely, characters that perform austerities (self sacrifices) in devotion to deities, or who are in service to various sages, are granted blessings by those deities and religious seekers. So while the heroes of the Mahbharata are told that they alone control their actions and thus, their salvation, their situations are often influenced by outside forces.
In summary, while the Vedas may be the ultimate authority in Hinduism, epics such as the Mahabharata and the Ramayana have had a more significant impact on the culture and behavior of Hindus.