Munna Bhai (Sanjay Dutt) falls in love with the voice of the Radio Jockey Jhanvi (Vidya Balan). He is finally given the chance to meet her when the radio station announces a trivia contest concerning the life and beliefs of Mahatma Gandhi. With the help of his friend Circuit, Munna Bhai formulates a clever scheme and wins the contest. In the process, however, Jhanvi is led to believe that Munna Bhai is a professor of history and subsequently asks him to present a lecture on Gandhi to a community of senior citizens which she takes care of at her home, Second Innings.
Munna decides that the only way out of this situation is to use the days prior to the lecture to learn as much as he can about Gandhi. He puts himself through an intensive period of study in a library devoted to the life and works of Gandhi. It is during this period of study that the image of Gandhi begins to appear to Munna Bhai. With Gandhi on his side, Munna not only wins Jhanvi's heart, but through a radio show also teaches others how to solve their problems using Gandhi's principles. Soon, however, Munna finds his own life falling apart until he learns to practice what he preaches. The problems he faces lead him to cultivate a new life based upon Gandhism, particularly non-violence and truth, which he calls Gandhigiri.