Based on the book A Long Way Home by Saroo Brierley, this is a somewhat fictionalized account of Saroo's both horrible and inspiring life story. He was born and raised in India, but at the age of five he got on a train he shouldn't have and ended up 1500 km from his home. Too young to find his way home, he lived on the streets of Calcutta for a short time and was eventually adopted by an Australian family. In 2012, after years of digging about in Google Earth, he identified his hometown - and eventually found his original family.
The first half of the movie concentrates on Sunny Pawar as the young Saroo in India, his poverty-stricken but loving family, and his terrifying dislocation. He's placed with the Brierleys (Nicole Kidman and David Wenham) in Australia, who take very good care of him. The second half of the movie has Dev Patel as the adult Saroo going to school in hotel management, where he encounters some other Australians originally from India and begins to question the shape of his life.
The first half of the movie is quite depressing, but the second half is astonishing and mostly very positive. The screenplay authors have made Saroo dysfunctionally obsessed with his quest to the point that he nearly loses both his girlfriend and his family over it (when all they want to do is help him). The real Saroo Brierley was quite a celebrity in both India and Australia when this was actually happening, and it seems to me that the writers have overplayed his obsession to make it more "interesting." Which seems unnecessary given very good performances all around that could have supported a somewhat lower key second half. In fact, I think it would have been better: I'm sure Saroo spent a lot of time on his search, but in every other respect the movie went to a lot of trouble to paint him as well adjusted, intelligent, and sane. Despite this questionable choice, the movie is mostly very good.
It looks like Dev Patel is finally turning into the good actor his résumé already suggests he is: I most recently saw him in The Man Who Knew Infinity in which he just looked stunned. He did a good job here.