The story is narrated by Rajbabu, who wants to write a book on the life of his friend, Sathya. Sathya, Rajan, Rajbabu, Mandiramoorthi and Haniff are college-mates in Madras. To celebrate their graduation, they go on a drive around the city in Haniff's car, singing about the ideals of life. The car crashes and Haniff dies, leaving his friends shocked. As time passes, Rajan, Rajbabu and Mandiramoorthi become corrupted in their pursuit of a materialistic lifestyle while Sathya vows to remain a righteous person. He meets Kanchana, a helpless young woman who has been raped and impregnated by a rich predator. She gives birth to a son, Kumar. Sathya marries her and raises Kumar as his own son. Kanchana's rapist later leaves his wealth to Kumar, but Kanchana refuses to take it. Sathya falls ill. His kidneys fail and his friends turn away when the doctor asks if any of them would donate their kidney to him. Sathya ultimately dies, but Kanchana continues to hold on to the values he taught her. Sathya's friends ultimately realize their folly and publish Rajbabu's book with the title Punnagai.