Eighteen-year-old Jamal Malik, an Indian Muslim from the Dharavi slum, is a contestant on the Indian version of Who Wants to Be a Millionaire?, and is one question away from the grand prize. However, before the ₹20 million (US$290,000) question, he is detained and tortured by the police, who suspect him of cheating. Through a series of flashbacks, Jamal recounts the incidents in his life that provided him with each answer.
Jamal's flashbacks begin when he is five years old. He manages to obtain the autograph of Bollywood star Amitabh Bachchan after jumping into an outhouse cesspit. Jamal's older brother Salim later sells the autograph. The incident is followed immediately by the death of the siblings' mother during the Bombay riots. As the brothers flee the riot they meet Latika, a girl from their slum. Salim is reluctant to take her in, but Jamal suggests that she could be their "third musketeer", a reference to the Alexandre Dumas novel The Three Musketeers. Although the brothers had read the novel in school, and referred to themselves as Athos and Porthos, it is revealed that they do not know the third musketeer's name. Latika ultimately joins the group despite Salim's initial refusal.