Whenever a danger of destruction of Religion was created, Almighty Lord Vishnu incarnated on the earth and restored Peace and Tranquility. In the Treta Yug, Lord Vishnu incarnated as Ram and killed Ravan the greatest tyrant of the time. In the Dwapur Yug Lord Vishnu once again was compelled to incarnate as Lord Krishna, the eighth son of Devaki and Vasudev who were put in prison by Devaki's brother King Kans of Mathura. Kans was challenged by the Divine voice (Akashwani) that Devaki's 8th son would kill him. Kans killed all the seven children of Davaki and awaited the birth of the 8th child. Immediately after the birth of 8th son, Lord Vishnu instructed Vadev to exchange and replace with the female child that was born in Gokul to Yashoda wife of Nand. Vasdev did so. Kans when informed of 8th birth tried to kill the child, who slipped from his grip and proclaimed that she was Goddess Yogmaya; Kans was Mad with fury and ordered massacre of all the newly born children.
With the lapse of time Lord Krishna grew up as smart child and gained love of the whole populace of Gokul. Child Krishna created a sensation by pulling down two trees and killing the gaintess Putna and two Pahlwans deputed by Kans to kill Krishna. Kans was stunned by the strength of Krishna.
Time rolled on and Krishna grew up into a young boy. The whole Gokul with his parents, Gawals Gopies, Cows were enchanted and haunted by his melodious flute and they all loved Him dearly. Radha the beautiful young Gopi loved him the most she used to dance to his magical flute. Krishna was Light for Radha and his absence Darkness. Their love was innocent, Divine and Platonic. But in the eyes of relatives and husband of Radha it was sin. They were jealous of Krishna. So the created opposition.
Krishna once defeated the Kalia Nagraj and when Kans tried to wipe out whole Gokul with tremendous storm that he created. Krishna lifted Goverdhan on His finger. Kans was defeated and frustrated. Kans then tricked and fooled his uncle Akrur who was great worshiper of Krishna with his crocodile tears and managed to invite Krishna in Mathura.
Krishna who was awaiting opportunity, started of Gokul with Akrur, leaving his parents, Gopals, Gopies and Radha; all weeping and sobbing. Parting was unbearable but Krishna left.
News of Krishna's arrival reached Kans. He was overjoyed. He ordered all possible ways to kill Krishna including the attack of intoxicated Elephant to crush Krishna... Did he succeed? How Lord Krishna removed the tyrant Kans, you can see on the Silver Screen.