Daag is the story of a man wrongly convicted of murder, on the run from the law. Sunil (Rajesh Khanna) is forced to leave behind his new bride Sonia (Sharmila Tagore), when he kills her would be rapist. In an attempt to establish a new identity Sunil meets Chandni (Rakhee) and ends up marrying her to give her illegitimate child his name.

A chain of events brings together Sunil and Sonia but the long arm of the law also catches up with him, on not just a murder charge but also his marriage of convenience, which is interpreted as bigamy.