“The Lunchbox” is now playing at Sundance Cinemas. PG, 1:45, three and a half stars out of four.

First of all, I want one of those lunchboxes. Instead of the suitcase-style boxes that Americans are used to, the Indian characters in “The Lunchbox” use an ingenious contraption made up of stacking cylinders, so you can put your vegetables in one cylinder, the rice in a second and the sauce in a third.

Secondly, I want the kind of lunch delivery system we see in Mumbai, where trainloads and trainloads of lunch containers somehow make it from home or a local restaurant to the right office every day. (One delivery man proudly says that “the Harvard people” have studied their system, which is a marvel.) In an opening sequence, we see one small green lunchbox moving from bike messenger to train to bike and then finally to work, and it’s hard not to draw a connection between that little lunchbox and its owner, nearly lost in the mass of people moving to and from work each day.

But if I want to see that lunchbox in America, I’m not sure I want to see an Americanized version of “The Lunchbox.” Writer-director Ritesh Batra’s confident first feature has a high-concept premise that would be easily translatable (such as the Japanese hit “Shall We Dance?”) but I think an American version would miss the subtleties of Batra’s film.

Irhan Kahn, who played the older Pi in “Life of Pi,” is Saajan, a lonely widowed accountant who has largely shut himself off from the pleasures of life. (At one point, we literally see him tell neighbor kids not to play near his yard.) His lunches come from a local diner, the same thing every day. But one morning, he unpacks his canisters to discover a mouth-watering collection of home-cooked Indian dishes. (When planning to see “The Lunchbox,” a pre- or post-show visit to “Taste of India” or one of Madison’s other fine Indian restaurants is a must.)

The lunch was actually meant for another man, Rajeev, and packed by his neglected young wife Ila (Nimrat Kaur). She’s like the wife in John Prine’s “Angel from Montgomery,” wondering how her husband can be at work all day, then “come home in the evening and still have nothing to say.” She’s trying to spice up their loveless marriage with, well, spices.

When her husband doesn’t comment on the meal, she slips a note into the next day’s lunch. Saajan sends one back (complaining that it was too salty), and soon the two start an unlikely epistolary friendship, sharing memories and details of their lives through the lunchbox letters.

This could be cloying in a “Lake House” sort of way, but instead “Lunchbox” is warm and sweet-tempered, as these two private people are able to reveal their inner selves in a way they can’t to those closest to them. The letters become a moment of quiet in a bustling Mumbai full of people — Saajan is often surrounded by people, whether in his big office or being jostled on the commuter train. But he couldn’t look more lonely. To see his eyes light up when he discovers a new note in his lunchbox, looking furtively around the cafeteria as if he doesn’t want his co-workers to see his joy, is a delight.

Khan is a wonderful actor, watchful and attuned, using the slightest of brushstrokes to let us know what’s going on behind Saajan’s tired old eyes. And Kaur is radiant as Ila, who also has a very funny rapport with her upstairs neighbor, who we often hear yelling down but rarely see.

“The Lunchbox” is a gentle film, and it is satisfyingly unpredictable in where it takes Ila and Saajan’s relationship, and how it changes them both. As Ila says late in the film, “Sometimes the wrong train will get you to the right station.” And sometimes the wrong lunchbox ends up in the right hands.