With his Oscar-winning film A Separation, the Iranian director Asghar Farhadi produced a gripping blend of family drama and whodunnit, wrapped in a socially acute document of Iranian society stuttering into the modern age.

His new film couldn't match it, and it doesn't need to. What The Past does do is confirm Farhadi as a consummate writer-director, a master at unravelling the psychological fissures of family life.

An Iranian, Ahmad (Ali Mosaffa), returns to Paris from Tehran to sign divorce papers with his French wife, Marie (Bérénice Bejo), four years after their separation. The couple have an easy rapport, with no evident rancour, and Ahmad gets on well with Marie's daughters from an earlier marriage, especially the eldest, the teenage Lucie (Pauline Burlet). The divorce ought to be a formality.

But things aren't quite right, as is made immediately apparent by the sleeping arrangements. Ahmad expects to check into a hotel, but Marie insists that he stay in their old home, a big house on the outskirts of Paris, despite the fact that new lover Samir (Tahar Rahim) and his son now live with her. The initial awkwardness between the two men is the least of it. It seems that the distraught Lucie is refusing to accept Samir in her mother's life; Marie wants her ex to find out why, and to intervene. Ahmad has been lured into a domestic storm that has nothing to do with him.

This is a very, very slow-burn affair. And in the first stages of the story, before the tension is ramped up, there is space to simply take pleasure in a delightful personality. Mosaffa's beautiful performance shows Ahmad to be a gentle, sensitive man, great with children, who adeptly navigates the awkward situation in which he finds himself, while dealing heroically with his own feelings of regret.

The reason his and Marie's marriage ended is only hinted at, but seems to have been more to do with Ahmad's cultural needs, his desire to return home, than any emotional breakdown between them. An initial moment at the airport, in which the pair communicate non-verbally through a glass partition, suggests both the barrier between them but also the fondness, and why she feels she needs him now.

Ahmad's people skills - not shared by the fiery Marie or morose Samir - make him an ideal, if reluctant arbiter in their crisis. And his presence slowly brings to the surface the secrets that are holding the new family back.

As the film's title suggests, Farhadi raises questions about our relationship with the past, the extent to which one should or can forget past love, and past pain, in order to move on. As one friend urges Ahmad, "cut … cut". If only it were that easy.

Still best known for her endearing role in the silent movie The Artist, Bejo (who won Best Actress last year at Cannes for The Past) offers here a vibrant, flesh-and-blood woman, and a very believable mixture of focused desire and wayward passions. Rahim (star of A Prophet) is perhaps the most mysterious of the three, his character weighed down by guilt (his own former wife lies in a long-term coma in a hospital), but who has a touching bond with his young son.

What The Past lacks, compared to its predecessor, is the laser-like understanding of the society in which it takes place; this is set in France, but the dynamics could be transposed to almost anywhere. What it shares is an attention to behavioural detail - the nuances of speech and intimacy, the domino effect of lies and misunderstandings - which is absolutely riveting.