Ammaiyappan Mudaliar, a government clerk, lives with his wife Godavari, sons Chidambaram, Siva and Bharathi, daughter Sarojini and Chidambaram's wife Uma. As Ammaiyappan struggles to manage the needs of his joint family with his meagre income, his two elder sons supplement it by contributing to the family kitty. He plans to get Sarojini married and invites the prospective groom's family for a prenuptial meeting.

The conceited Sarojini rebukes the guests, and declares her plan to marry her colleague Peter Fernandez. The entire family initially opposes her decision as they are Hindus and Peter is a Christian. Ammaiyappan meets with Peter's father Albert, and realises that they are genuine humans, and both families ultimately agree on the wedding. Ammaiyappan meets and apologises to the rejected groom's father. Impressed with Ammaiyappan and his family, he offers to get his daughter Vasantha married to Siva. Both weddings take place happily. Uma, pregnant with her first child, goes to her maternal home for delivering the child.

Bharathi fails his twelfth grade exams for the fourth time, and is beaten severely by Siva. Later, he vows to pass within a year, and seeks help from Vasantha who is a graduate. But within weeks, Vasantha struggles to adjust to the new environment as she gets little privacy to be with Siva as Bharathi seeks her help for tutoring even at late night and the family chores are keeping her busy in the day. Unable to cope, one early morning she returns to her father, without informing anyone. Sarojini returns the same morning to Ammaiyappan after a fight with Peter as she prioritises socialising over domestic responsibilities. Siva visits Vasantha's father, who is berating Vasantha. The duo then walk home, when Siva tells her to either wait a few more months, for privacy, or let go of him. Vasantha returns to Siva's home, while Siva leaves for work.

That same day, Chidambaram halves his monthly contribution to the family, citing Uma's absence. An argument erupts between him and Ammaiyappan. Upset with Chidambaram's tightfistedness on spending money for his siblings and parents, Ammaiyappan orders him to vacate the house. In turn, Chidambaram demands the ₹18000 he had spent for Sarojini's wedding with interest for him to leave. Deeply insulted, Ammaiyappan draws a white-line splitting the house into two halves and announces that no one from either side should cross it or communicate with members of the other side. He also declares Chidambaram's family has to reside in one side, and Ammaiyappan will pay net sum of ₹20000 within 12 months for Chidambaram to vacate the house. The only person with access to both sides is the maidservant Kannamma.

Uma later returns with her child and is shocked with the developments at home. She is sad to see Sarojini back and an unhappy Vasantha. Uma takes the help of Albert and Kannamma to reunite Sarojini and Peter, and advises Siva to take Vasantha on a sojourn to rejuvenate his relationship with his wife. Bharathi also passes his exams. Though the problems are sorted out, Uma is upset that her brothers-in-law are still abiding by their father's order and not talking to her. Kannamma cajoles Ammaiyappan not to be very adamant and forgive his son.

Chidambaram, meanwhile, ends up spending more money per head for his small family after the separation, than what he allocated when they were together with the extended family. So he reconsiders his decision to live separately and decides to reunite with his father. However, Uma disagrees as once the bonding is broken for the sake of money, it cannot be restored exactly to its original state. She states that they may not rejoin for the sake of money and any bonding should be based on unconditional love. She recommends living separately but on talking terms, celebrating weekends and festivals as the best way to live peacefully, for which her husband agrees, and they move out the same evening.