North Gate of Stupa #1, Sanchi
From the lower architrave (rear). Vessantara is the previous life of the Buddha, just before his incarnation as Siddhartha. The jataka (story) is continued from the front of the architrave, where king Vessantara has given away his kingdom and all his possessions to those in need. Proceeding from right to left, the king and his family appear in their wilderness hut. In the next scene over, Vessantara gives away his children to a cruel Brahmin, who sells them into slavery. (The king's follower aims an arrow at the Brahmin, but is dissuaded by Vessantara from harming him.) To the left of this, Vessantara, never one to do a thing halfway, gives away his wife. Finally, the god Indra acknowledges his selflessness and provides a happy ending by restoring his family and possessions.
The point of this story is that a bodhisattva (a Buddha in the making) must be prepared to renounce everything that he holds dear, and to immediately let go of any human attachment. The cause of suffering is desire, and one can achieve true peace only by relinquishing all claims of the self.