Eleven-year-old Ferdinand bashes 11-year-old Bruno with a stick, removing two teeth. And what do the victim’s parents do? Act very, very civilised, inviting their counterparts for a parley that they hope will lead to an apology from the offender. But the author is Yasmina Reza, the caustic Parisian who wrote Art, and from the moment you see Janet McTeer and Ken Stott being painfully nice to Ralph Fiennes and Tamsin Greig you know that there are cruelties and lies, rocks and cliffs, in the dramatic offing.

With Matthew Warchus directing these superb performers and Christopher Hampton translating, the effect is tense, edgy and funny. The problem, as the title hints, is that Reza wants us to see her molehill as a mountain. Her subjects come to embrace African genocide, conflict resolution, restorative justice and the moral nature of us human animals – and, though she might retort that microcosms may imply macrocosms or acorns signify oaks, the play cannot bear such weight.

Janet McTeer’s Véronique, who is writing a book about Darfur, begins by exuding high-minded generosity. The trouble is that the visitors basically do not believe that their son is to blame. It gradually emerges that Annette (Tamsin Greig) thinks that Ferdinand was provoked by a kid who then sneaked on him. It’s more immediately obvious that Alain (Ralph Fiennes) feels that everyone but self-righteous prigs should expect boys to be savages. That’s the way “the god of carnage” created us: an idea that is anathema to Véronique, who thinks that evolution has changed even the male of the species. Indeed, Fiennes’s hilariously offhand Alain, who is a lawyer, makes it clear that the whole business bores him and spends much of the session on his mobile phone, urging colleagues to hide the truth about the side-effects of a drug sold by a company that he represents. Inevitably discord arrives and escalates, and not only between the two couples.

Annette drowns her husband’s phone in a flower bowl. Véronique rages at Ken Stott’s Michel, a self-confessed “uncouth” whose sins include chucking the family hamster into the street and calling marriage “the most terrible ordeal God could inflict on man”.

A bottle of rum is unwisely opened. Emotions intensify. Alliances shift this way and that. Misogyny, racial prejudice and homophobia enter the equation. Stott sneers at his wife’s “Sudanese coons”. McTeer’s virtuous façade cracks when Greig, who has earlier maddened her by vomiting on a favourite book, calls Bruno “a snivelling little poof” who deserved his beating.

Despite a touching end, we are not left feeling that Reza has much faith in love, altruism or you and me. If we cannot solve playground rows how can we halt the world’s atrocities? Sometimes I felt that Reza’s scepticism, rather than human logic, was manipulating her characters and determining their misbehaviour. But again and again I found myself delighted by her incisive observation, her acerbic wit, her shrewd humour – and her stunning cast.

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