Canadian-born director Mehta shows a keen eye for the mean streets of his ancestral homeland in this small, heartfelt story of a man's search for his missing son.
The father is Mahendra (Tailang), whose itinerant work repairing zips for a few rupees barely keeps his wife, Suman (Chatterjee) and two children in their one-room slum shack.
The son is the title's Siddharth - Siddhu to his family - a 12-year-old sent to work in the Punjab factory of a distant relative (Srivasta).
When Siddhu does not return for Diwali, Mahendra is told he has absconded, but soon he is dealing with the possibility of a much darker truth. One character will tell him, "missing children are big business".
As Mahendra spends money he does not have and time he cannot afford on his quest, the mystery unfolds alongside the story of a man struggling to deal with the endless, intimidating complexity of the teeming world beyond his tiny neighbourhood.
He's unsure how to use his mobile phone and he has no photograph to provide with a missing-person report to officials, who are hostile because he has violated child-labour laws ("You people never learn," says a cop; "Business was slow," he pleads, "we needed the money.")
A glimmer of a clue is a report of a place called Dongri, which hangs like a false sunrise over the gloom. But Mehta is too clear-eyed to pretend there is a happy ending to a story rooted in such tragic realism. The film's most chilling line has a boy of Siddhu's age telling Mahendra that "maybe [Siddhu] got lucky and left this world".
Siddhu (Khan), unseen after the first reel, is nevertheless a ghostly presence throughout, thanks to Mehta's canny decision to use the same young actor in small cameo roles, which seem to mock the father's search. Each false lead draws him deeper into debt and confusion, and the film is unsparing in its depiction of the interlocking exploitations that characterise life on the margins.
In its quasi-Dickensian scenes of daily life, Siddharth has the precision of ethnography; in its storytelling, it channels the neo-realist aesthetic of the great Satyajit Ray, though a syrupy score undermines some of the effect.
It's a striking and polished addition to the catalogue of recent cinema set in India and is a bracing antidote to the spit-shined glibness of Slumdog Millionaire.
Verdict: Street thriller
Cast: Rajesh Tailang, Tannishtha Chatterjee, Irfan Khan, Amitabh Srivasta, Khushi Mathur
Director: Richie Mehta
Rating: M (adult themes) In Hindi with English subtitles
Running time: 97 mins