Saroo and his older brother, Guddu, live with their mother and little sister in Khandwa, India. They do not have much money and Guddu takes Saroo with him to work lifting bales of hay. On the way there, Saroo falls asleep on a train platform bench and gets separated from his brother. The five-year-old Saroo ends up on a train to Calcutta, and when he arrives there, he has no way of finding his way back home.
In Calcutta, Saroo narrowly avoids being sold into sexual slavery and eventually ends up homeless, before getting placed in an orphanage. Soon enough, a woman named Mrs. Sood tells him that a couple from Australia want to adopt him. Mrs. Sood teaches Saroo English so that he will be able to fit in in his new country. In 1987, he moves to Hobart, the capital of Tasmania, and becomes the adoptive son of Sue and John Brierly.
Fast forward two decades, and Saroo moves to Melbourne where he studies hotel management. There, he starts dating Lucy, a white American, but most of his friends are of Indian heritage. At dinner one night he tastes a dessert called jalebi, which he remembers enjoying as a child. He tells his friends that he isn't actually from Calcutta; he's actually been a lost child for the last twenty years. One of his friends tells him to use Google Earth to find his hometown, and Saroo begins to search for his village.
Saroo's adoptive mother, Sue, is distraught about his adoptive brother, Mantosh, who is addicted to hard drugs and struggling. Saroo goes home to visit with her and talks to her about his adoption. He learns that she always wanted to adopt out of choice, and that it was not a biological imperative for her.
After stalling for several months, and never quite managing to pinpoint his little village on Google Earth, Saroo spots some rock formations that were near where his birth mother worked. He finds the region he comes from and finally admits to Sue that he has been looking for his former home. She is wholeheartedly supportive of his search.
Saroo goes back to his hometown and finds an interpreter who speaks English so that he can communicate with his birth mother. She always believed that he was still alive and that he would one day come to find her, and so never left the village. He reunites with his sister, but learns that his brother Guddu was hit by a train and killed the night that he went missing.