18-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$300,000) question, he is detained and tortured by the police, who suspect him of cheating because of the impossibility of a simple "slumdog" with very little education knowing all the answers. Jamal recounts, through flashbacks, the incidents in his life which provided him with each answer.

Jamal's flashbacks begin with his managing, at age five, to obtain the autograph of Bollywood star Amitabh Bachchan, which his brother Salim then sells, followed immediately by the death of his mother during the Bombay riots. As they flee the riot, the brothers meet Latika, a girl from their slum. Salim is reluctant to take her in, but Jamal suggests that she could be the "third musketeer", a character from the Alexandre Dumas novel The Three Musketeers (which they had been studying—albeit not very diligently—in school), whose name they do not know.

The three are found by Maman—a gangster who tricks and then trains street children into becoming beggars. When Salim discovers Maman is blinding the children in order to make them more effective beggars, he flees with Jamal and Latika to a departing train. Latika fails to board the train as Salim purposefully lets go of her hand, in revenge for an extremely embarrassing prank that Latika had played on Salim. This resulted in her being recaptured by Maman. Over the next few years, Salim and Jamal make a living travelling on top of trains, selling goods, picking pockets, working as dish washers, and pretending to be tour guides at the Taj Mahal, where they also steal people's shoes. At Jamal's insistence, they return to Mumbai to find Latika, discovering that she is being raised by Maman to be a prostitute, soon to fetch him a high price as a virgin. The brothers rescue her, and while escaping Maman they shoot him to death. Salim then manages to get a job with Javed—Maman's rival crime lord. Back at their room, Salim orders Jamal to leave him and Latika alone. When Jamal refuses, Salim draws a gun on him, whereas Latika persuades Jamal to obey his brother and go away.

Years later, Jamal, now a Chaiwala in an Indian call centre, searches the centre's database for Salim and Latika. He fails in finding Latika, while succeeding in finding Salim, now a high-ranking lieutenant in Javed's crime organisation. Jamal reproaches Salim, who then pleads for forgiveness, and offers him to stay in his luxurious apartment. Jamal later bluffs his way into Javed's residence to reunite with Latika but as he professes his love for her, Latika asks him to forget about her. Jamal nevertheless promises to wait for her every day at five o'clock at the VT station. Latika attempts to meet him there, but is recaptured by Javed's men, led by Salim, and once Javed moves to another house, outside Mumbai, the two again lose contact. Jamal becomes a contestant on Who Wants to Be a Millionaire?, knowing that she watches the show regularly. Much to the consternation of Prem Kumar, the show's host, Jamal becomes a wonder across India. During a break before Jamal answers the penultimate question, Kumar tries to fool him by feeding him the wrong answer whilst in the men's room. After the break Jamal uses the 50/50 lifeline, leaving Kumar's answer and one other, but chooses the other one.

The episode ends before the final question can be asked, and Kumar leads Jamal out of the studio where he is immediately forced into a police van. After an initial beating, the police inspector listens to Jamal's explanation of how he reached each answer. Finding all of them "bizarrely plausible", he allows him back on the show. At Javed's safehouse, Latika sees Jamal on the news and Salim, in an effort to make amends for his past behaviour, gives Latika his mobile phone and car keys, and asks her to forgive him and to go to Jamal. Latika is reluctant out of fear of Javed, but agrees and escapes. Salim fills a bathtub with money and sits in it, awaiting for Javed and his men as they realize that he let Latika free. Jamal's final question is the name of the third musketeer in The Three Musketeers, which he never learned. Jamal uses his "Phone-A-Friend" lifeline to call Salim's 'phone, as it is the only phone number he knows. Latika answers the phone, and, while she does not know the answer, tells Jamal that she is safe. Relieved, Jamal arbitrarily picks Aramis, the right answer, and wins the grand prize. Javed hears Latika on the show and realises that Salim has betrayed him. He and his men break down the bathroom door but Salim kills Javed before he is shot, gasping, "God is great". Soon thereafter, Jamal and Latika meet on the platform at the railway station and they kiss.

Screenwriter Simon Beaufoy wrote Slumdog Millionaire based on the Boeke Prize-winning and Commonwealth Writers' Prize-nominated novel Q & A by Vikas Swarup. To hone the script, Beaufoy made three research trips to India and interviewed street children, finding himself impressed with their attitudes. The screenwriter said of his goal for the script: "I wanted to get (across) the sense of this huge amount of fun, laughter, chat, and sense of community that is in these slums. What you pick up on is this mass of energy."

By the summer of 2006, British production companies Celador Films and Film4 Productions invited director Danny Boyle to read the script of Slumdog Millionaire. Boyle hesitated, since he was not interested in making a film about Who Wants to Be a Millionaire?, which was produced by Celador. Then Boyle learned that the screenwriter was Beaufoy, who had written The Full Monty (1997), one of the director's favourite British films, and decided to revisit the script. Boyle was impressed by how Beaufoy wove the multiple storylines from Swarup's book into one narrative, and the director decided to commit to the project. The film was projected to cost $15 million, so Celador sought a US film distributor to share costs. Warner Independent Pictures stepped up with $5 million and won rights to the picture.

Gail Stevens came on board to oversee casting globally. Stevens had worked with Boyle throughout his career and was well known for discovering new talent. Meredith Tucker was appointed to cast out of the US. The film-makers then travelled to Mumbai in September 2007 with a partial crew and began hiring local cast and crew for production in Karjat. Originally appointed as one of the five casting directors in India, Loveleen Tandan has stated, "I suggested to Danny and Simon Beaufoy, the writer of Slumdog, that it was important to do some of it in Hindi to bring the film alive They asked me to pen the Hindi dialogues which I, of course, instantly agreed to do. And as we drew closer to the shoot date, Danny asked me to step in as the co-director." Boyle then decided to translate nearly a third of the film's English dialogue into Hindi. The director fibbed to Warner Independent's president that he wanted 10% of the dialogue in Hindi, and she approved the change. Filming locations included shooting in Mumbai's megaslum and in shantytown parts of Juhu, so film-makers controlled the crowds by befriending onlookers. Filming began on 5 November 2007.

In addition to Swarup's original novel Q & A, the film was also inspired by Indian cinema. Tandan has referred to Slumdog Millionaire as a homage to Hindi cinema, noting that "Simon Beaufoy studied Salim-Javed's kind of cinema minutely." Boyle has cited the influence of several Bollywood films set in Mumbai. Deewaar (1975), which Boyle described as being "absolutely key to Indian cinema", is a crime film written by Salim-Javed based on the Bombay gangster Haji Mastan, portrayed by Bollywood superstar Amitabh Bachchan, whose autograph Jamal seeks at the beginning of Slumdog Millionaire. Anil Kapoor noted that some scenes of the film "are like Deewaar, the story of two brothers of whom one is completely after money while the younger one is honest and not interested in money." Slumdog Millionaire has a similar narrative structure to Deewaar. Satya (1998), written by Saurabh Shukla (who plays Constable Srinivas in Slumdog Millionaire), and Company (2002), based on the D-Company, both offered "slick, often mesmerising portrayals of the Mumbai underworld" and displayed realistic "brutality and urban violence." Boyle has also stated that the chase in one of the opening scenes of Slumdog Millionaire was based on a "12-minute police chase through the crowded Dharavi slum" in Black Friday (2007), adapted from Hussein Zaidi's book of the same name about the 1993 Bombay bombings.

Boyle has cited other Indian films as influences in later interviews. The rags-to-riches, underdog theme was also a recurring theme in classic Bollywood movies from