'Idhayakkani' is the story of the kind-hearted Mohan (MGR) whose theory of life is 'equality' and who puts it in practice by sharing equally the income of his estate with his labourers. He has another passion - he wants to serve the country - and for that, he has joined the police force as an officer. He meets a needy girl, Lakshmi, and on the advice of and elderly domestic help, he permits her to stay in his house. Tongues start wagging and Mohan decides to marry the girl. However, his happiness is short-lived. He gets the shock of his life when he comes to know that his wife is involved in the murder of a leading scientist. What is the truth? Did Lakshmi murder the scientist or is she innocent. If so, then who is the culprit? How does Mohan go about finding the truth?