Based on a true story, a five-year-old boy named Saroo leaves his village in rural India to tag along with his brother on a work assignment. When his brother leaves him alone and never returns, Saroo searches for him on a train, which ends up being non-operational and doesn’t stop until he’s in Calcutta, a thousand miles away. He wanders the streets until he is eventually brought to an orphanage and later, adopted by a loving couple in Australia. But as an adult, he is haunted by the idea that his biological mother and brother will never know he is safe. After someone tells him about Google Earth, he spends two years scouring the map until he is finally able to locate the village he used to live. He travels back there and reunites with his mother and sister but learns his brother was killed (in post-script, we learn his death was shortly after Saroo saw him for the last time).
Saroo (Sunny Pawar as a child, Dev Patel as an adult) is a very young boy who lives with his impoverished mother, brother and sister where they all make money doing manual labor. One day, Saroo follows his brother Guddu (Abhisek Bharate) to the train station where the latter is looking for work. Arriving at night, Guddu leaves a sleeping Saroo on a bench in the station then leaves to find someone. When Saroo wakes up, his brother has failed to return. He looks for him on a train and falls back asleep expecting Guddu to find him. When he wakes up he finds the train (actually a decommissioned passenger train) is moving and tries to find a way out of the carriage to no avail. After travelling for days, eventually ending up hundreds of miles away from where he started, Saroo is able to leave the train when it reaches Calcutta. Saroo, who speaks Hindi, finds that many of the people (who speak Bengai) don't understand him and he has no idea how to get back home. After foraging on the streets and avoiding child abductors, Saroo is discovered and handed into the authorities. Unfortunately, Saroo mispronounces the name of his town so no-one knows where he came from. As a result, Saroo is put into an orphanage.
Three months later, Saroo is told that he has been adopted by an Australian couple, John (David Wenham) and Sue (Nicole Kidman). He is taught to speak english before moving to Tasmania where he meets John and Sue who accept him lovingly into their household. Saroo quickly settles into his new life, however a year later John and Sue adopt another boy named Mantosh (Keshav Jadhav as a child, Divian Ladwa as an adult) who has much more serious problems and is prone to self-abusing.
Twenty years later, Saroo is a well-adjusted adult who deeply loves his adoptive parents is about to leave for Melbourne to do a hotel management course. However there is a divide between Saroo and Mantosh, Saroo angry for the stress that he puts their parents (especially Sue) under. While on the course, he meets Lucy (Rooney Mara) and the two begin a relationship, but on a night out with friends he is reminded of his former home and begins to think about the life he left behind and the emotional agony his mother and brother must have gone through when he went missing. He is introduced to Google Earth (then a new technology) and learns that if he investigates he may be able to find where he came from.
The search takes years as Saroo only has fuzzy childhood memories to rely on, and it causes him to disconnect from his family (as he worries that John and Sue may consider him ungrateful for looking for his natural family) and Lucy. He is about to give up, however when aimlessly using Google Earth he finds some geological markers he recognizes. Following them, he finds the station where he fell asleep and eventually location the real name of the neighbourhood (which he thought was a town) he'd been mispronouncing. He tells Sue what he'd been doing and, instead of being ungrateful like he'd worried, she completely understands his desire to find where he came from and promises her full support.
Saroo flies to the town of Khandwa and heads to the neighbourhood of Ganesh Talai which is where he lived as a child. He is upset to find his former house is now gone, and shows his photo to a man in the street who leaves in a hurry. Believing his family must be gone, Saroo is then gestured by the man to follow him. Doing so, Saroo has an emotional reunion with his sister and biological mother as the town looks on, but is heartbroken to be told Guddu is dead. Even so, he is still happy beyond words to be reunited with his family and the movie ends with Saroo walking along the train tracks he had followed Guddu on as a child.
Captions state that Saroo had been missing for twenty five years before he came home and had travelled over 900 miles. Guddu had killed by a train on the night both boys had gone to the station, but Saroo's mother never gave up hope her youngest son might return so never left the town. It is also revealed that Saroo had been mispronouncing his name his entire life; his actual name was Sharu which was Hindi for 'Lion'. There then follows pictures of the real family as well as video of him introducing the real Saroo and Sue as he introduces his adoptive mother to his biological mother where the three embrace each other.