This documentary is a little miracle of gentleness and humanity: a vérité study of three Mexican brothers – Diego, Rodriguez and Bruno. They come together to look after their much-loved 93-year-old grandmother América after she has injured herself falling out of bed, an accident that was the end result of negligence by their father Luis (her son) who has been sent to prison.

There is great tension between the boys, which at one stage erupts into a bizarre playground scuffle in the kitchen, but great love also, and it is beguiling to think that América in some unconscious sense sacrificially contrived her own accident to bring the family together and heal their wounds.

The movie itself arose from a happy accident. Directors Erick Stoll and Chase Whiteside went to Mexico intending to make a documentary about US tourists there, and got talking to Diego, a street theatre entertainer. From there, the two film-makers followed their instincts and found a better story – an object lesson in how great documentary-making often involves a readiness to tear up the original plan and start afresh.

América is the natural star of the film – as she puts it, “a startled star”. She is the bewildered but courageous figure of sublime, benign beauty. She is nourished by the infusion of love she receives from the devoted Diego and her dignity and poise are utterly impregnable, even when the boys have to get her out into the backyard and administer an enema.

The film has real artistry in the way it is put together. It has the potency of a sympathetically fabricated kind of realist fiction, a guided-reality effect that is completely absorbing. The coda leaves you with an irresistibly sweet sadness.