Dev (Akshay Kumar) and Ajay (Saif Ali Khan) are two good for nothing boys living in the city of Mumbai. Both are orphans brought up by a chronic alcoholic (Avtaar Gill). They are small timers dreaming of big times and coming up with several schemes to become rich. One such idea by Ajay is to simply find a rich girl...make her fall in love and then marry her. After the marriage he intends to become an in-house husband and live off his wife's riches, and enjoy life forever. Dev and Ajay zero down on a girl who they think is very rich, Sharmilee (Raveena Tondon).
Sharmilee, with similar idea, in reality is a two bit con woman who eventually falls in love with Dev. One day Ajay picks a young man's pocket. The man Mohan (Ravi Kissan) decides to chase Ajay, during which he is killed in an accident. When Dev and Ajay open the wallet they find a letter through which they realise how important that money was for Mohan's family. Feeling pangs to guilt they decide to go to Mohan's home in Badlapur and return the money to his family.
On meeting Mohan's father Dinanath Tripathi (Anupam Kher), a school master and mother (Moushami Chatterji), they are unable to tell the truth. Mohan's family accepts them as their own, their son's best friends. In course of time they become the entire family and village's beloved. Ajay also finds his love in the same village, a girl called Mansi (Sonali Bendre).
Will the family ever come to know about Mohan's death? When they do, how will they treat Dev and Ajay, whom they have come to love as their own sons? What will be the Keemat that Dev and Ajay pay in order to regain their love and trust?