Daag is the story of Sunil (Rajesh Khanna) and Sonia (Sharmila Tagore) who meet in college and fall in love and get married against their elders wish. One fine day Sunil and Sonia escape the rain to take shelter in Dheeraj's (Prem Chopra) house where Dheeraj tries to force himself upon Sonia when Sunil intervenes and kills Dheeraj in a fit of anger. Sunil is sentenced to jail while a pregnant Sonia leaves the city. A few years later Sonia after giving birth to a son gets a job in a school where she meets Chandani (Rakhee) and realizes that her husband is no one else but Sunil and they have a daughter. What happens is a tale of twists and turns and many revelations that leave Sonia and the audiences spellbound.