In 1986, five-year-old Saroo lives with his elder brother Guddu, his baby sister Shakheila, and his mother in Khandwa, India. Guddu and Saroo steal coal from freight trains to trade for milk and food. Saroo accompanies Guddu to work overnight, and they arrive at a nearby train station, where Saroo falls asleep on a bench and is unable to find Guddu upon waking up. He searches for him on an empty train, only to fall asleep in one of the compartments and awake sometime later to find the train in motion and the doors locked. After several days, the train arrives in faraway Calcutta, where Saroo does not understand the local Bengali language. He tries to obtain a ticket home, but the attendant does not recognise the name of his village, which Saroo says is "Ganestalay".[4] He spends the night in the station with some street children, but is forced to flee when a group of men try to kidnap them.
Saroo continues to wander around the city before meeting Noor, a seemingly friendly woman who takes him back to her apartment and tells him that a man named Rama will help him find his way home. Saroo escapes, sensing that Noor and Rama have sinister intentions, and evades Noor when she chases after him. After two months of living near the Howrah Bridge, Saroo is taken to the police and placed into an orphanage when authorities are unable to trace his family.
An advertisement about Saroo is placed in several local newspapers, but no one responds, though an Australian couple has become interested in adopting him. Saroo is taught basic English and moves to Hobart, Tasmania, in 1987, under the care of Sue and John Brierley, where he slowly starts to settle into his new lifestyle. A year later, they adopt another boy, Mantosh, who has trouble adjusting to his new home and suffers from rage and self-harm.
Twenty years later, Saroo, now a young man, moves to Melbourne to study hotel management and starts a relationship with American student Lucy. During a meal with some Indian friends at their home, Saroo reveals that he is not from Calcutta and was separated from his birth family more than twenty years ago, and his friends suggest he use Google Earth to search for his hometown. Saroo begins to look, and, overwhelmed by the thought of the pain he imagines his family in India must have been feeling ever since he was lost, he becomes obsessive and gradually withdraws from Lucy and his adoptive family, though he does not tell his adoptive family about his search for his biological family.
Eventually, after hearing his adoptive mother is not doing well because he has pulled away and Mantosh (who has substance abuse issues) has gone missing, Saroo visits her to apologise, and learns that she is not infertile, as he had always assumed, but wanted to help children in need through adoption. Feeling overwhelmed by how much more ground is left to cover in his search, one night Saroo recognises the rock formations where his mother worked and finds the area where he lived: the Ganesh Talai neighbourhood of the Khandwa district. He finally tells his adoptive mother about his search, and she fully supports his efforts.
Saroo returns to his hometown and, with the help of a local English speaker, has an emotional reunion with his biological mother and sister, but he is heartbroken to learn that Guddu was hit and killed by a train the same night they were separated. His mother remained in the village for the 25 years since he went missing because she never gave up hope that he would return one day. In addition to mispronouncing the name of his village, Saroo learns that he also mispronounced his own name as a child, as his biological parents named him not "Saroo", but "Sheru", meaning "lion".
