In 2006, 18-year-old Jamal Malik, an Indian Muslim from the Juhu slum of Mumbai, is a contestant on Kaun Banega Crorepati. Before answering the final ₹20 million question, he is detained and tortured by the police, who suspect him of cheating. Through a series of flashbacks, he recounts the incidents in his life that provided him with each answer.
At five years old, Jamal obtains the autograph of Bollywood star Amitabh Bachchan after jumping into a cesspit. Jamal's elder brother Salim later sells the autograph. Their mother is killed during the Bombay riots. While fleeing the riot, the brothers briefly encounter a child dressed up as Rama, with a bow and arrow in their right hand. Having escaped the riots and taking shelter from the rain, the brothers 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 which the brothers had learned about in school. The brothers refer to themselves as Athos and Porthos but do not know the third musketeer's name.
The three children are found by Maman—a gangster who trains street children to become beggars. After learning that Maman is blinding the children to make them more effective beggars, Salim escapes with Jamal and Latika. The brothers successfully board a moving train, but Latika is unable to keep up. Salim grabs her hand but purposefully lets go as revenge for pulling a prank on him, leaving her to be recaptured by Maman. For the next few years, Salim and Jamal travel on top of trains, making a living by selling goods, pickpocketing, washing dishes, and pretending to be tour guides at the Taj Mahal. At Jamal's insistence, they return to Mumbai to find Latika and discover that Maman is raising her to be a prostitute. The brothers rescue her, Salim shooting Maman dead. Salim gets a job with Javed—a rival crime lord. In their room, Salim orders Jamal to leave him alone with Latika, presumably to sexually assault her. When Jamal refuses, Salim draws a gun on him, and Latika persuades Jamal to leave.
Years later, Jamal, now working as a chaiwala in a call centre, learns that Salim is a high-ranking lieutenant in Javed's crime organisation. Jamal confronts Salim, who pleads for forgiveness. Jamal then sneaks into Javed's residence and reunites with Latika. Although he professes his love for her, she tells him to forget her. Despite the refusal, Jamal promises that he will wait for her every day at five o'clock at Victoria Terminus. Attempting to meet him there, Latika is captured by Javed's men, led by Salim. They scar her face while driving away. Jamal loses contact with Latika and in a final attempt to reach her, he becomes a contestant on Kaun Banega Crorepati, knowing she watches the show.
Jamal plays extremely well and becomes popular across India, much to the dismay of the show's host, Prem Kumar. Kumar attempts to trick Jamal by feeding him the wrong answer to the penultimate question. However, Jamal answers correctly, raising suspicion of him cheating.
When the episode ends, Jamal is arrested. After an initial beating, the police inspector listens to his explanation of how he knew each answer. The officer believes Jamal and allows him to return to the show. Latika sees that Jamal was arrested on the news. Feeling guilty about his past behaviour, Salim gives Latika his phone and car keys, asking her to forgive him. After Latika leaves, Salim fills a bathtub with money and sits in it, waiting for Javed to realise what happened.
For the final question, Jamal is asked the name of the third musketeer. Jamal admits to not knowing the answer and uses his "Phone-A-Friend" lifeline to call Salim because it is the only phone number he knows. Latika answers and tells Jamal that she is safe, but does not know the answer. Javed hears Latika on the show and realises that Salim betrayed him. He and his men break down the bathroom door. Salim kills Javed before being shot and killed by the gang. Relieved about Latika, Jamal guesses and picks the first answer, Aramis. He is correct and wins the grand prize. Jamal and Latika meet on the platform at the train station, kiss, and dance to the song "Jai Ho" along with all the other characters.
