Jamal Malik, a young man who hails from the worst slum area of Calcutta, India, is now in the hot seat on Who Wants to Be a Millionaire?, one question— one snippet of trivia—away from winning twenty million rupees. Jamal is faring so well that producers are beginning to get anxious; how can a boy from the slums be doing so well on the gameshow?
On the eve of Jamal's final question, the host has him kidnapped by the police and brought in for questioning. After various forms of torture do not work, they go with him question by question and ask him how he knows the answers. Each question pertains to a different step of his journey in life, and he starts from the beginning.
At an early age, Jamal and his older brother, Salim, were orphaned in the Bombay riots, and had to make money however they could alone in the slums. They meet a young girl, Latika, who joins them, becoming their "third musketeer." The three are later caught and exploited by local gangster Maman, who kidnaps orphans and forces them to perform and sing on the street in order to make him money. When Salim witnesses Maman blind a child to make him a more sympathetic beggar, he and Jamal escape, but 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 the process, Salim kills Maman. Afterwards, he threatens his brother and kidnaps Latika for himself.
Years pass. Unable to forget about Latika, Jamal is now working at a call center. After using a computer in his office to look up Salim's phone number, Jamal arranges a time to meet. Not long into their reunion, Jamal learns that Salim's employer is a slumlord, Javed Khan, and is most likely the one keeping Latika. He follows Salim to Javed's compound, and sure enough, finds Latika there. Even though Latika tells Jamal to forget about her, he tells her 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 actually shows up at the train station at the agreed-upon time. Soon after they spot each other on the platform, however, Salim arrives with some of Javed's "heavies" with him, and they kidnap Latika, throwing her in a car that drives away.
Back in the present time, Jamal explains to the police questioning him that the only reason he went on the show in the first place was because he thought Latika might be watching, as he saw her watching it at Javed's. 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 Alexandre Dumas novel?"
Back when Jamal and Salim were close to each other, they had said that they were Athos and Porthos, but they never actually gave Latika a moniker when she joined them, simply calling her "the third musketeer." Jamal admits to the gameshow host that he doesn't know the answer, but he uses his final "phone a friend" to call the only person whose telephone number he knows by heart, his brother Salim. 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, kinder self, and helped her to escape Javed's clutches. She doesn't know the answer to the question, but tells Jamal something that he wants to know far more—that she is safe. He then correctly guesses the name of the Musketeer from the four choices given (Aramis), winning the jackpot. 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.
