Jamal Malik (Dev Patel), un joven huérfano que vive en una barriada pobre de Bombay, decide presentarse a la versión india del concurso: "¿Quién quiere ser millonario?". Ante la sorpresa de todos, Jamal responde correctamente a todas y cada una de las preguntas. ¿Cómo es posible que un chico cómo él sea capaz de conocer todas las respuestas? Cuando Jamal está a punto de responder a la última pregunta, la que le hará ganar 20 millones de rupias, la policía lo detiene y se lo lleva para interrogarle. Jamal deberá explicar por qué conocía las respuestas, teniendo que recurrir para ello a relatar diferentes momentos de su vida, que además ayudarán a desvelar la verdadera razón de su participación en el concurso.

The story of Jamal Malik, an 18 year-old orphan from the slums of Mumbai, who is about to experience the biggest day of his life. With the whole nation watching, he is just one question away from winning a staggering 20 million rupees on India's "Kaun Banega Crorepati?" (2000) (Who Wants To Be A Millionaire?) But when the show breaks for the night, police arrest him on suspicion of cheating; how could a street kid know so much? Desperate to prove his innocence, Jamal tells the story of his life in the slum where he and his brother grew up, of their adventures together on the road, of vicious encounters with local gangs, and of Latika, the girl he loved and lost. Each chapter of his story reveals the key to the answer to one of the game show's questions. Each chapter of Jamal's increasingly layered story reveals where he learned the answers to the show's seemingly impossible quizzes. But one question remains a mystery: what is this young man with no apparent desire for riches really doing on the game show? When the new day dawns and Jamal returns to answer the final question, the Inspector and sixty million viewers are about to find out. At the heart of its storytelling lies the question of how anyone comes to know the things they know about life and love.