In 2006, the story unfolds with eighteen-year-old Jamal Malik (Dev Patel), a young Indian Muslim hailing from the Juhu slum in Mumbai, who finds himself as a contestant on the popular game show Kaun Banega Crorepati. Just moments prior to answering the final question worth Rs 20 million, he faces a harrowing interrogation by police constable Srinivas (played by Saurabh Shukla), who suspects him of foul play. The inspector, convinced that no educated individual could reach such heights on the show, subjects Jamal to torturous methods, including electric shocks, all in an effort to force a confession.
Amidst this chaos, Jamal reflects on the pivotal moments from his life that led him to each answer, utilizing a series of flashbacks. Living in the notorious slum of Dharavi alongside his brother Salim and their mother, Jamal's childhood is marked by significant experiences. At a tender age of five, determined to meet his idol, he jumps into a cesspit to get an autograph from Bollywood legend Amitabh Bachchan. However, Jamal’s elder brother Salim ([Madhur Mittal]) later sells that precious autograph, introducing Jamal to the reality of exploitation.
As they navigate their turbulent lives, tragedy strikes when their mother ([Sanchita Choudhary]) is killed during the devastating Bombay riots. In an attempt to escape the chaos, the brothers encounter a child donning the guise of Rama, symbolizing hope amidst despair.
While seeking refuge from the rain, they stumble upon Latika (Freida Pinto), a girl from their slum. Though hesitant, Salim eventually agrees to let her join them as their "third musketeer," a playful nod to Alexandre Dumas's classic The Three Musketeers, which they had studied in school.
As their tumultuous journey continues, the children find themselves under the control of Maman (Ankur Vikal), a menacing gangster who exploits street children by forcing them into begging. The brothers eventually escape from Maman’s clutches, but not without a price – they are forced to leave Latika behind.
Time passes, and the brothers resort to various means of survival, including pickpocketing, selling goods atop trains, and pretending to be tour guides to naïve tourists. Jamal's tenacity drives him to return to Mumbai to find Latika, who tragically has been caught in a web of crime and is being groomed for a life of prostitution.
As Jamal's life takes unexpected turns, he confronts Salim, who is now associated with a rival crime lord, Javed ([Mahesh Manjrekar]). In a tense moment, Salim betrays Jamal’s trust, forcing him to leave Latika to protect her.
Years later, while working as a Chaiwallah in a call center, fate leads Jamal back to his old life as he learns Salim’s new status as Javed's lieutenant. With determination, Jamal uses his knowledge about British pop culture and a call center software to locate his brother, hoping to save Latika from her grim fate.
Determined to reunite with Latika, Jamal appears as a contestant on Kaun Banega Crorepati, fully aware that she is an avid viewer. His journey garners immense popularity across India, much to the annoyance of the show’s host, Prem Kumar ([Anil Kapoor]). In a final twist, Jamal's desperate quest for answers sees him navigate the complexities of his past, culminating in a tension-filled finale where he must rely on the loyalty of his brother Salim and the enduring connection with Latika.
Amidst chaos, Jamal confronts deep-seated fears and familial ties. In a moment of suspense, with the ultimate question involving the identity of the third musketeer, Jamal employs his "Phone-A-Friend" lifeline, leading to a dramatic revelation that intertwines their fates. The film concludes with a heartwarming reunion between Jamal and Latika at the train station, reaffirming love amidst turmoil, where they finally embrace and share a long-awaited kiss.
