Wang Xiaoshuai navigates an ocean of sadness in this film. It can finally be watched only through a blur of tears and with a terrible, futile need to reach into the screen and hug the two ageing lead characters. So Long, My Son is an epic generational drama of two families in China, from the 1980s to the present day; directed and shot with clarity and calm, audaciously structured in terms of flashback and flashforward – and acted superbly.

There is also touch of melodrama in it, if such a thing can be said to exist in the unshowy walking pace of Wang’s storytelling style, with some influences from classic Japanese family drama perhaps, and gestures of soap-operatic melancholy, largely in the repeated (and unexpected) use of the plaintive tune Auld Lang Syne. This is a film that doesn’t signpost its relevant facts very emphatically and you have to stay alert for shifts in the timeline, and for important details that are only revealed later. But, once you have mentally readjusted away from traditional linear expectations, this movie opens up like a flower.

It is about the terrible burden of grief, rage and guilt, and the greater burden of forgiveness; it is also about an emotional wound that only gets worse with the years. That wound has been inflicted on two levels: by the ordinary, arbitrary heartbreak of life and by the malign agencies of the Chinese state, with its draconian one-child policy to control population and boost economic growth, begun in the late 70s and not completely abandoned until 2015.

Liyun (Yong Mei) and Yaojun (Wang Jingchun) are an obedient, hardworking married couple in the big city with an eight-year-old boy Xingxing. Their best friends are fellow factory hands Haiyan (Liya Ai) and her husband, who have an eight-year-old son of their own, Haohao; their kids are best friends. But, when Liyun gets pregnant with an (illicit) second child, she discovers just how much of an apparatchik party-zealot her friend and neighbour Haiyan actually is. She reports Liyun to the authorities and gets her dragged off to the hospital for an abortion she doesn’t want, while Yaojun impotently rages at the government and at himself for failing to stand up for Liyun. And then Haohao boisterously chivvies and bullies Xingxing into going swimming at a dangerous reservoir with the other, wilder neighbourhood kids, despite Xingxing’s timid complaints that he can’t swim. The catastrophic result (all the more painful for never being explicitly shown) inflicts a crippling psychic blow to all four adults and to Haohao, who is to grow up with a need to go into the medical profession and save lives.

As the new millennium dawns, fate provides a new twist to the suppressed guilt suffered by Haiyan and her family by making them wealthy in the new Chinese world of adventure. Meanwhile, wretched, lonely Liyun and Yaojun move away, to a remote coastal town where they adopt a boy, Xing (Roy Wang), who senses that he is second-best and becomes a tearaway delinquent, breaking his adoptive parents’ already shattered hearts. Moreover, Haohao’s glamorous aunt Moli (Xi Qi) forms a tendresse for Yaojun, which creates its own refinement of pain.

At the centre of the film are the wonderfully compassionate and tender performances from Yong Mei and Wang Jingchun as the ageing, lonely pair whose unexpressed agony, by the end of this film, feels unbearably intimate. For me, the tragedy of their relationship is revealed in the closing act when they return to their hometown, grey old little country mice that they now are, silently goggling at all the glitzy new buildings and video ad hoardings – signs of that commercial triumph that the one-child policy was there to deliver.

Yaojun appears to wave cheerily at something outside the car. It turns out to be a statue of Mao waving, almost dwarfed amid the steel and glass, and Yaojun turns his placid smile at his wife as if to explain: that is what I was waving back to, you see? Isn’t it silly? And how absurd to shed a tear now after all this time?

Apart from everything else, this film reveals a terrible, simple truth: those who have endured the terrible agony of losing a child are not a separate tribe (that is: separate from the luckier ones) destined or earmarked for tragedy from the beginning. Neither are they people who have endured this blow in return for the expectation of some mysterious compensatory gift from the cosmos. Their fate is arbitrary and they are just like us; our current happiness used to be theirs.

So Long, My Son is a piercingly, profoundly moving picture that peels and exposes the senses.

• So Long, My Son is released in the UK on 6 December.