The gameshow "Who Wants To Be A Millionaire" is not just part of the fabric of popular culture in America and the United Kingdom; now the pheomenon has spread to the other side of the world, and Jamal Malik, a young man who hails from the worst slum area of Calcutta, India, is now in the hot seat, one question, one snippet of trivia away from wining twenty million rupees - roughly four hundred and thirty thousand US dollars at the time of the show. For anybody this would be a life changing amount of money. A person could definitely call the office and say they weren't coming in for a while. But for a boy from one of the most horrendous slums in the world, it's not just a life altering amount of money. It's almost incomprehensible. How in the world could a boy from a slum know the answers to the questions that can net him winnings like that?
This is exactly the question that the makers of the television show were asking themselves and they involved the local authorities, who arrest Jamal for cheating on the gameshow. However, Jamal didn't learn the answer to the questions by reading books; he learned from experience, and he begins to tell the story of his life, which reveals exactly how and where he gained his knowledge.
At an early age, Jamal and his older brother, Salim, were orphaned, and had to make money however they could alone in the slums. They meet a young girl whose situation is not really any better than their's; eventually, Latika joins them, becoming their "third musketeer". The three are later caught and exploited by local gangster Maman, who is just like Fagin from Dickens' "Oliver Twist" and forces orphans to perform and sing on the street in order to make him money. Although Jamal and Salim are able to escape, Latika is not so lucky. For a few years the brothers survive on the street, keeping on the move, but Jamal cannot seem to forget Latika and decides that they should go back to see if they can find her. They rescue her and in doing so, Salim kills Maman This changes him. He threatens his brother and takes Latika with him.
Years pass. Unable to forget about Latika, he is now working at a telemarketing company. Thinking about it, he realizes that the only way to find Latika is to contact his brother. They meet, for the first time in years, and very quickly Jamal realizes that his brother's employer is a slumlord, and is most likely the one holding Latika. He follows Salim to Javed's compound, and sure enough, finds Latika there. An idea begins to germinate in his head, even though he doesn't know it yet; whilst he is talking to Latika, "Who Wants To Be A Millionaire" is on the television in the background. He tells Latika that he will be waiting for her at the train station every day until she joins him. Latika is frightened that Javed will catch them and tells Jamal that he must leave. She later shocks him when she is actually waiting for him at the train station; he never believed that she would join him. Predictably, though, Salim arrives with some of Javed's "heavies" with him, and they stop Latika leaving.
Back in the present time, Jamal explains to the police quesitoning him that the only reason he went on the show in the first place was because he thought Latika might be watching it, as it was always on at Javed's compound. The police release him, and he is returned to the television studio in time to answer the last question; what is the name of the third musketeer in the Alexander Dumas novel?
Back when the brothers were close to each other, they had said that they were Athos and Porthos, but they never actually gave Latika a moniker like that when she joined them; they had simply called her "the third musketeer." Jamal admits to the gameshow host that he doesn't know the answer; but true to his "look on the bright side" character, he decides to play anyway. He uses his final "phone a friend" he calls the only person whose telephone number he knows by heart - his brother Salim. It's not particularly likely that Salim will help him, given their more recent encounters, but Jamal is shocked to find that when he calls, it is not Salim who answers the phone call, but Latika. Salim has reverted to his former, nicer self, and escaped Javed with her. They live in a safehouse. She doesn't know the answer to the question but tells Jamal something that he wants to know far more than the name of the third musketeer - that she is safe. He then correctly guesses the name of the Musketeer from the four choices given (Aramis). He wins the jackpot. Unfortunately, Salim does not fare so well, and chooses to sacrifice himself to protect Latika and his brother. Jamal and Latika meet once again at the train station, knowing that this time they won't be parted again. The movie ends with the pair performing an elaborate Bollywood dance sequence.