Ammaiyappan Mudaliar, [Visu], a government clerk, lives with his wife Godavari, [Kamala Kamesh], their children Chidambaram, [Raghuvaran], Siva, [Vagai Chandrasekar], and Bharathi, [Khaja Sharif], as well as their daughter Sarojini [Ilavarasi] and Chidambaram's wife Uma [Lakshmi]. In a tight-knit, busy household, Ammaiyappan grapples with the demands of a growing joint family on a modest income, while his two elder sons help supplement the finances.
He plans to arrange Sarojini’s marriage, inviting the prospective groom’s family for a prenuptial meeting. The conceited Sarojini rebukes the guests and declares she intends to marry her colleague Peter Fernandez [Dilip]. The family initially resists the match, wary of a Hindu family pairing with a Christian suitor. Ammaiyappan then meets Peter’s father, Albert [Kishmu], and discovers that both families are decent people. The two families eventually come to an understanding about the wedding, and Ammaiyappan thoughtfully apologizes to the rejected groom’s father.
Impressed by Ammaiyappan and his kin, Albert offers to have his daughter Vasantha [Madhuri] marry Siva, and both weddings proceed with happiness. Uma, who is pregnant with her first child, travels to her maternal home to deliver the baby, hoping the family can remain united in the long run.
Bharathi fails his twelfth-grade exams for the fourth time, and Siva disciplines him, leading Bharathi to vow to succeed within a year and seek help from Vasantha, a graduate. Yet Vasantha struggles to adapt to the new environment, finding little privacy to be with Siva, while Bharathi seeks tutoring late at night and the family chores keep her busy. Unable to cope, she returns to her father one morning without informing anyone. Sarojini returns the same morning after a clash with Peter, prioritizing socializing over domestic duties. Siva visits Vasantha’s father, who criticizes Vasantha, and the couple then debate whether to wait a few more months for privacy or let go of their arrangement. Vasantha eventually returns to Siva’s home, while Siva heads to work.
That same day, Chidambaram halves his monthly contribution to the family, citing Uma’s absence, and an argument erupts between him and Ammaiyappan. Frustrated by Chidambaram’s tightfistedness regarding money for his siblings and parents, Ammaiyappan orders him to vacate the house. Chidambaram counters by demanding the ₹18,000 he had spent on Sarojini’s wedding, with interest, to leave. Deeply insulted, Ammaiyappan draws a white line across the house, declaring that neither side may cross it or communicate with the other. He sets out that Chidambaram’s family must reside on one side, while Ammaiyappan will pay a net ₹20,000 within 12 months to secure his departure. The only person with access to both sides is the maid Kannamma [Manorama].
Uma returns with her child and is shocked by the rancor at home. She worries over Sarojini’s return and an unhappy Vasantha. With help from Albert and Kannamma, Uma tries to reunite Sarojini and Peter, and she counsels Siva to take Vasantha on a brief trip to rejuvenate their relationship. Bharathi also passes his exams. Although the immediate problems seem settled, Uma remains troubled that her brothers-in-law still adhere to their father’s decree and refuse to speak with her.
Kannamma gently tries to calm Ammaiyappan and urges him to forgive his son. Meanwhile, Chidambaram discovers that separating the family spreads costs thinner per head than when they lived together, and he begins to reconsider his choice. He contemplates rejoining with his father, but Uma argues that once a bond is broken for money, it cannot be restored exactly as it was. She suggests living separately yet on talking terms, celebrating weekends and festivals together as the healthier path, and Ammaiyappan ultimately agrees, with the family moving out that very evening.
