Ever since the first Homo sapiens emerged from her cave on an idle morning and wondered “Who am I?” the human race has been inventing and re-inventing answers to that question. The latest people to pick up on that have been the scientists, as they start to tackle the great question of consciousness.

Their interest has been caught by dramatists, with On Ego opening last month at the Soho Theatre and Imposters opening tonight at the Union Theatre in Southwark. This surely must be the first London season that two plays have appeared in which a major part is played by the rare and strange Capgras’ Syndrome – in which the victim of a brain injury believes their nearest and dearest have been replaced by near-identical imposters. (Meanwhile in New York there’s a whole theatre festival on the subject, in which this play is included.)

Yet, when you think about it, what better way to tackle issues of identity than this? Certainly the American playwright Justin Warner has used this as a fruitful way to approach what is a common tale – a family and a marriage under strain when the children have grown up and the holes at the heart of a long marriage are suddenly exposed.

The narrator of the tale is the youngest son, Andrew (Edd Hunter), a young post-graduate science student, alternately cockily sure of himself then consumed by doubt, a characterisation that many will recognise in his class. He’s sometimes a bit of an irritating know-it-all, but usually endearing, although lacking in depth. Andrew takes some massive emotional shocks with barely a shrug, and comes to some major life decisions without sign of a tremour.

The real emotional core and heart of the play – and indeed the family – is his mother, Peggy, a fine performance by Joyce Springer, who manages to be both ditzy and deep in a single moment. She’s been the victim for 24 years of emotional abuse from her husband, Frank, a frustrated and angry salesman, and her (Catholic) church. Continually told she is stupid, Peggy has learnt to play up the stereotype. The only dream left to her – the summit of her (unachievable) ambition – is to be the hand model in the Palmolive dishwashing adverts.

Meanwhile her priest, Father Ryan (Graham Elwell), is telling her not to worry, God will wipe out all of her sins, wearily repeating his mantras as most of his attention is focused on his racing tips. When she suddenly is revealed to have taken these seriously, he is shocked into stammering incoherence.

The crisis arises after her older son, Vincent (Aidan Synnott) – a stay-at-home layabout – suffers the brain injury that brought on the Syndrome, and rejects both of his parents as imposters. This leads to the arrival of Paul (Denis Quilligan), now a high-powered psychiatrist, and arrogant and disconnected human being. He is, it seems, an “old friend of the family”, if now, with his European sophistication, convinced that he’s far above them. Quickly it emerges, however, that he was rather more than that for Peggy, at least on one boozy New Year’s Eve.

So, who belongs here, in “the family”, if it is indeed that, and who does not? Who knows who they are? Have they been living a genetic, a personal or an emotional lie for decades?

That sounds heavy, but this is a script with a ready wit, played with a light, but never flippant, touch. So the audience laughs readily every time that Peggy exclaims “Jesus Christ”, before adding “Sorry” to the heavens and crossing herself, and when Frank comments on her lack of knowledge of Paul’s career by saying “He does cutting edge research. They don’t have it in Readers Digest.”

The director, Aoife Smyth, makes good use of the warm, intimate space of the Union, with a lovingly detailed, realistic set that, with layers of accretion, from fridge magnets to old jars of jam, suggests the decades of backstory behind the family, yet allows for rapid, seamless scene changes.

So, unlike On Ego, the science is firmly wrapped in layers of traditional theatre. No one who was turned off science at school need be discouraged from seeing this show.

Imposters continues at the Union Theatre until January 28. Tickets are £10/£8.