Afamily of four and a therapist tiptoe around one another in a long, empty room with a two-way mirror at one end. We could be in a hospital or a youth detention centre. Their movement could not feel more agonisingly physical, nor the mirror more exposing. Yet the characters barely move from chairs that are far apart in a space sketched out by designer Naomi Dawson in strip-lighting and scaffolding, which runs through the centre of the audience. The room is in fact a theatrical mirage, constructed by writer Ruby Thomas from the sort of asymmetrical communication that occurs when something really bad has happened and nobody knows who to blame or what to do about it. In a series of gripping group therapy scenes, we find out about the family breakdown that has brought student Sam to this place, but it’s not until the final stretch that anyone puts a name to what he has done. Sam’s father (Jonathan McGuinness) sits in stony silence, while his doula mother (Martina Laird) witters half-baked therapeutic platitudes and his eager-faced younger sister (Ashna Rabheru) is excruciatingly ignored. Everyone – as therapy-speak would put it – is entirely attentive. Evasions that at first seem comic and stereotypical become increasingly damaging and particular, bouncing Ragevan Vasan’s wired Sam between surly and sad, as Paul Keating’s gentle therapist struggles to open up space for self-reflection. The symbolism of the title is lightly touched on with fleeting mentions of bonobos and swifts (Sam is a zoology student). Is this a therapy room or a human zoo, with invisible spectators? It certainly makes you question what theatre is. There’s no plot to speak of, and I was astonished to see a fight director credited: it’s just words. But in Lucy Morrison’s coiled-spring production, it is also so much more. The fizz of the electric lighting as the scenes shift in time; the reconfiguration of the characters between sessions, so that you never see them all face on at the same time; the slow reveal of something you should have worked out but somehow didn’t – that’s pure theatre. At Hampstead theatre, London, to 26 March.