Nalandhaanaa is a family drama that explores the purity of relationships and the emotional weight of traditional values. The story begins with a unique challenge: Ramu (played by Prabhu), a company executive, bets his cynical friend (played by Y.G. Mahendran) that a man can maintain a purely sibling-like bond with a woman. Ramu chooses Raji (played by Menaka), the granddaughter of his office clerk, and accepts her as his foster sister. To support her, he even poses as her family member to rent a house and eventually arranges her marriage to her cousin, Babu (played by Bhaskar). Ramu is also quietly loved by Bama (played by Rajalakshmi), and their wedding is soon planned.

Just as Ramu prepares for his own wedding with Bama, tragedy strikes. Babu is killed in a horrific accident shortly after the wedding. Knowing how much Raji cherishes her status as a bride, the family and Ramu struggle with the agonizing decision of whether to reveal the truth. Raji, sensing a dark shadow but unaware of her husband's demise, meets with a fatal kitchen accident involving a stove.

On her deathbed, believing her husband is still alive and recovering, Raji begs Ramu not to let Babu know about her impending death, wishing to die with the peace of being a "Sumangali" (a woman whose husband is alive). The film concludes as a tribute to the traditional sentiment where a woman considers dying before her husband as her greatest honor, framed through the selfless protection of an "elder brother."