The rains only come at the end of this film, but there is no drenching emotional release to go with them; the weather is more complicated. Cambodian-British film-maker Hong Khaou, who directed the gentle tale of love and loss Lilting, has created a thoughtful, deeply felt movie of great sweetness, unfolding at an unhurried pace. It is about a homecoming that isn’t quite a homecoming, a reckoning with something not exactly there, an attempted reconciliation with people and places that can’t really be negotiated with.

Henry Golding (the sleek young plutocrat from Crazy Rich Asians) plays Kit, a young British-Vietnamese man who has come out to the old country on a mission to make some sense of his family history. He left Saigon when he was six years old with his brother, mum and dad; they wound up in Hong Kong and from there went on to Britain. It is charming and genuinely touching when Kit remembers as a child witnessing his late mother telling a consular official: “I want to come to England because I love the Queen very much.”

The plan is that Kit’s brother (and his wife and two sons) will join him in Vietnam later and they will later decide where to scatter the ashes of their parents. They evidently died a while back, some years apart, without ever having returned to Vietnam or expressed a wish to do so – and Kit is unsure of the symbolism of this. But while he is in Saigon, Kit has an online hookup with Lewis (Parker Sawyers, who memorably played Barack Obama in Southside With You), the son of a troubled Vietnam vet. Like Kit, he brings his own unacknowledged baggage to Vietnam.

Kit’s most fraught reunion is with Lee, who was his best friend when he was six – a quietly excellent performance by David Tran. Lee is reasonably pleased to see Kit after all this time: he introduces him to his daughter and to his elderly mother. At first, Kit makes a good impression on the mother with his presents of chocolates, sweets and whisky – but there’s a wince-making moment when he presents her with a water-filtration gadget that he realises, a fraction of a second too late, is an unsubtle insult about the quality of their drinking water. Lee has a modest mobile phone business and there is a difficult history of how his family got the money for this commercial venture. Lee has something reproachful and even angry in his attitude to the coolly self-possessed young Kit, whose family got out of the country and is now evidently prosperous enough to go travelling like this, while most Vietnamese of his age can’t. Later, a young art curator in Hanoi called Linh (Molly Harris) will tell him she can’t go travelling because her family sacrificed so much for her education in Vietnam.

Most importantly, and perhaps with a touch of cruelty, Lee is to challenge Kit’s memory of how and why he got out of Vietnam. Kit remembers the drama and the heartache of how they all left together as a family, with a kind of solidarity. But Lee tells him it wasn’t quite like that, and this revelation sows a seed of doubt and anxiety that silently flowers throughout the movie.

Later in Hanoi, Kit meets Linh, who ushers in the film’s most unexpectedly charming scene: her parents have a business “scenting” tea with flowers such as lotus blossom (an activity that exasperates Linh because only old people drink scented tea like this). Kit sits in on a scenting session with Linh and her folks, in which they sit around, preparing the flowers by hand. “Are you bored yet?” asks Linh drily – and I laughed, because I wasn’t bored. It’s weirdly fascinating.

Some months ago, Spike Lee released his powerful Da 5 Bloods about Vietnam vets returning to the country to confront their demons. Much as I admired that film, I concede the justice of those who say that it overlooked the experiences of Vietnamese people. This movie addresses those ideas more directly, and engages with their stories. Its intelligence is a tonic.

• Monsoon is in cinemas from 25 September.