The Mahabharata is, with the Ramayana, the greatest Hindu epics. It tells the story of the war between two clans, the Kauravas and the Pandavas. The Pandavas receive help from Krishna, the 8th avatara of Vishnu.
The Mahabharata differs however fondamentally from the Ramayana in that the plot is much less manichean. The distinction between good and evil, dharma and adharma, clear in the Ramayana, is very confused in the Mahabharata. Good men perform evil actions and the plan of the gods remain obscure till the end.
The Mahabharata tells the story of the cataclysmic events that inaugurated the Kali-Yuga, the age of darkness and confusion in which we live. It contains not only the famous Bhagavad-Gita but also a discourse of Bishma on the foundations of kingship in Ancient India.
The adaptation by Brook is a master piece that captures well the ambiguity of the epic poem. The 6-hour film is available online and it is worth watching: