18-year-old Jamal Malik (DEV PATEL) is one question away from winning 20 million rupees on India's version of "Who Wants to be a Millionaire?" a far cry from growing up as an orphan in Mumbai's slums. Yet, that very past has cast doubts on his ability to know the answers, resulting in him being interrogated by the Inspector (IRRFAN KHAN) and his brutal goon who want to beat a confession of cheating.
Jamal won't budge, however, and as the Inspector plays back questions posed by the show's host, Prem (ANIL KAPOOR), the young man recounts his past life experiences that ultimately lead to his knowledge of the answers. We then flashback to when young Jamal (AYUSH MAHESH KHEDEKAR) and his slightly older brother, Salim (AZHARUDDIN MOHAMMED ISMAIL), end up orphaned during a massacre of Muslims, including the murder of their mother.
They eventually befriend another orphan, Latika (RUBINA ALI), and then end up in an orphanage run by Maman (ANKUR VIKAL), a brutal man who employs various kids as panhandlers. The brothers manage to escape, ending up at the Taj Mahal where they grow up into young teenagers, Jamal (TANAY HEMANT CHHEDA) and Salim (ASHUTOSH LOBO GAJIWALA), living off the streets and hustling tourists. Feeling badly that Latika didn't escape with them, Jamal convinces his brother to return to Mumbai to rescue Latika (TANVI GANESH LONKAR), but even that eventually goes amiss.
Years later, Jamal works at a call center and wonders whatever happened to Latika (FREIDA PINTO) and Salim (MADHUR MITTAL), discovering that they're now involved in different ways with Javed (MAHESH MANJREKAR), a local, powerful crime boss. As Jamal tries once more to rescue her, the story goes back and forth between the past and present where the 18-year-old tries to prove his innocence to get back to the show and its last question.