Tanu Weds Manu. Tanu (Kangana Ranaut) and Manu (R. Madhavan) have been married for four years now and they live abroad. They have fallen out of love and both visit a team of psychiatrists to see if there is a possibility to improve matters but as bad luck would have it, Manu is taken to be suffering from a mental disorder, by the team of doctors and, therefore, put in a mental asylum. The bold Tanu leaves him to fend for himself and returns to her parental home in India. She informs his friend, Pappi (Deepak Dobriyal), who lives in India, about Manu's state.
Pappi reaches London and gets Manu released from the mental asylum and the two of them return to India. One day, quite by chance, Manu sees a look-alike of Tanu and mistakes her for his estranged wife. But she turns out to be Kusum alias Datto (Kangana Ranaut), who hails from Haryana and studies in Delhi. She is an athlete. Manu falls in love with Kusum due to her looks which resemble Tanu's. Slowly but surely, Kusum also reciprocates even though she knows that he is a married man. Meanwhile, Manu has sent Tanu a legal notice more with the intention of initiating talks with her for a rapprochement. Tanu's lawyer, without her knowledge, replies to that by sending Manu a legal notice for divorce. Tanu has, in the meantime, touched base with Raja Awasthi (Jimmy Shergill) who was madly in love with her and had wanted to marry her.
As it turns out, Kusum alias Datto is to marry Raja Awasthi who has chosen her solely because she resembles Tanu. All hell breaks loose when Tanu learns of Manu's plans to marry Kusum. She confronts both, Manu and Kusum, but is unable to stop the marriage.