The film opens as Jamal Malik, a former street child from the slums, is tortured by two Mumbai policemen. As he is tortured, we learned that Jamal has been a contestant on the show Who wants to be a millionaire? hosted by Prem Kumar. He has already won 10 million rupees and has made it to the final question, for 20 million, scheduled for the next day. Kumar had told the police that he suspects Jamal of cheating.
The rest of the film is organized into two mixed plots. In one plot, the police ask Jamal how he came to know the answer to a question, and Jamal replies by recounting an incident in his life, told in flashbacks. In the other plot, the flashbacks tell the story of Jamal, who grew up to be a low-paid tea-vendor, his brother, Salim, who grew up to become a gangster, and Latika, who grew up to become a kept woman. It is also a love story between Jamal and Latika.
The story of Jamal's life includes obtaining the autograph of Bollywood star Amitabh Bachchan, which was how he answered the first question, and then the death of his mother during anti-Muslim violence. Salim and Jamal meet Latika, another street child from their slum and, although Salim is reluctant to take her in, Jamal persuades him, suggesting that she could be the “third musketeer". The three are eventually discovered by Maman while they are living in the trash heaps. Maman is a gangster who pretends to run an orphanage in order to "collect" street children to beg money for him. Salim feels the danger and they escape just in time, and flee, jumping onto a departing train. Latika catches up and takes Salim's hand, but Salim purposely lets go, and she is recaptured by the gangsters.
The two boys make a living travelling on top of trains, selling goods, picking pockets, and cheating naíve tourists by pretending to be tour guides at the Taj Mahal. Jamal insists that they return to Mumbai to find Latika. They eventually locate her, discovering that she has been raised by Maman to become a prostitute and sell her virginity. The brothers attempt to rescue her. In the resulting conflict, Salim draws a gun and kills Maman. After the escape, Salim gets a job with Javed a rival crime lord. Salim returns to the room where the three are staying, asserts his older brother status, and orders Jamal to leave so that he can have sex with Latika. The shocked and furious Jamal attacks Salim, who draws his gun and threatens to kill his brother. Latika intervenes and tells Jamal to leave.
Years later, the three kids have grown up and have followed different paths in life. Jamal is certain that he loves Latika and will try hard to find her back. He also wants to know about his brother. How will life have treated them? Will the two lovers meet again? Will a Slumdog millionaire be able to escape from poverty and find his true love?
In a fast-paced ending we'll follow Jamal's frentic run to meet his fate.