The film begins with a riddle from the Vikramathithan-Betal story, "Ennudaya appa yarukku maamanaro, avarudaya marumagalin appa en maganukku maamanar. Appa avarukkum enakkum enna uravu?" (My father is father-in-law to someone; that person's daughter-in-law's father is my son's father-in-law. What is the relationship between him and me?) which gets answered at the end in the film.
Prasanna (Kamal Haasan), son of a rich widower Mahendran (Major Sundarrajan), is a young student who indulges in several revolutionary activities, much against the wishes of his father. During one such activity, Mahendran helps the police in getting Prasanna arrested, which widens the already existing rift between them. After his release from prison, Prasanna opts to stay away from his home town Bangalore and reaches Madras. One day, Prasanna is beaten up for abusing passengers in a car and is abandoned on the road. Famous Carnatic vocalist MR Bhairavi (Srividya), returning home after a music performance, notices him lying unconscious on the road, takes him home, and gets him treated. At her request, Prasanna continues to live in her house even after he recovers and slowly becomes very passionate about her, despite her being older by several years.
Before Prasanna's arrival, Ranjani (Jayasudha), who believes she was adopted by Bhairavi, learned that she is Bhairavi's own daughter born out of wedlock. Unable to digest Bhairavi's hypocrisy, she runs away from home and lands in Mahendran's house at Bangalore while trying to sell some items. She ends up staying in his house for a few days at his request. Impressed by his love for his deceased wife and longing for his son Prasanna who left him, and his commitment to help her, she expresses a desire to marry him, though he is old enough to be her father.
Both Bhairavi and Mahendran initially reject the proposals of Prasanna and Ranjani, quoting their age difference and other situations, but with perseverance, Prasanna and Ranjani are able to convince them on how their respective marriages can work. Bhairavi is forced to accept Prasanna's proposal when he gives up his revolutionary activities and takes up music for her sake. When Prasanna is happy at getting Bhairavi's acceptance, Bhairavi's ex-lover Pandian (Rajinikanth), father of Ranjani, who is ailing from blood cancer, turns up to meet her and apologize. Prasanna does not allow him to meet Bhairavi and takes him away for treatment at his place. He informs Pandian of his plan to marry Bhairavi and seeks his cooperation by not meeting her. Pandian agrees.
Mahendran, seeing an advertisement issued by Bhairavi about her missing daughter Ranjani, comes to meet Bhairavi and ends up seeing his son Prasanna's photo in the same house. He understands the situation through the maidservant. He advertises about his missing son Prasanna which Bhairavi sees, so she insists that Prasanna should get his father's consent before marrying her. Both of them land up at Mahendran's place. Mahendran introduces Ranjani (daughter of Bhairavi) as Prasanna's future mother since she is his future wife. Now the riddle is explained. Prasanna's wife will be the mother of his father's wife! This means Ranjani will be the mother-in-law to her own mother and Mahendran will be the son-in-law of his own son! The real answer to the riddle is: for a husband, the relation is his wife and for the wife, the relation is her husband! Mahendran demands how this riddle is going to be cleared in their case. The result is shown in Bhairavi's climax concert in which she sings the song 'Kelviyin Nayakane lndha Kelvikku Padhil edhayya?' (What is the answer to this question, the hero of the question?) Ranjani changes her mind midcourse and joins her mother by singing at the concert. Observing this, Prasanna also changes his mind and joins his father. When all these issues are cleared, Pandian sends a note to Bhairavi in the middle of the concert. Bhairavi searches for him after the concert and finds him dead with a note in his hand, wishing her well with Prasanna. She removes her 'Kungumam' (vermilion on forehead) to declare herself a widow and leaves with Ranjani, while Prasanna joins his father.
[from the book Pride of Tamil Cinema: 1931 to 2013 by G Dhananjayan, Blue Ocean Publishers, 2014]