Blackbird is a really fine piece of work, a dirty, brutal little show, with an assurance and maturity from its director and performers that stands out like a bitter pitch-black diamond from a crowded theatre scene. It deserves its subterranean setting in Pembroke’s cellars: that murky, black-walled space will take us right down to the sordid depths of human behaviour, forcing us to watch something we'd rather never think about. And then the play does us an extra cruelty by making us think about it, too.

Charlie Merriman's mid-fifties office worker Ray has been tracked down by Nisha Emich's Una, the girl he had sex with when she was twelve, some fifteen years after the event. Watching a play about paedophilia is a difficult ask to begin with; but when you are brought to see the perpetrator as a human being, as he explains his self-loathing and years of penitent agony and his horribly real love for the younger Una, the experience becomes something akin to an endless silent scream mixed with a disorientated grasping for solid ground. Like all the best theatre, you split your time between asking what you're supposed to think and asking if we should care what we're supposed to think.

The performances are superb at finding the nuance in this sorry dance between condemnation and sympathy. Merriman's is an intelligent and utterly controlled portrayal, starting with a banal awfulness as he stutters his weasly protests to this unwelcome reminder of his past, his wheedling nervous energy bouncing off the rock-steady Emich like a two-bit scumbag attempting to appease an avenging angel. And then the performances begin, inexorably, to change gears. His body is brittle and broken, his shoulders forever rounded and arms hanging awkwardly off his beaten frame; hers is lithe and poised, her silkenly cruel voice delivering the slow-burning exposition as a relentless hammer of terrible truth.  Both in their unique ways convey a deep-seated damage startling for actors of their years.

There is no overplaying of emotion, despite the extraordinary circumstance – the performers have captured playwright David Harrower's rhythms with elegance, building to a breathless conclusion without ever overplaying their hand. The audience can never be certain whether the experience is corrosive or cathartic.  The play cannot help but humanise a paedophile, and by having to look the great bogeyman of our time straight in his awful face we gain a fleeting clarity on how little we actually think about the issue. When Ray refers disgustedly (and without self-awareness) to a “Them”, those other criminals of his ilk who he considers genuinely evil, we see a gradation of badness that no tabloid headline will ever begin to admit.

Whatever moral judgements we might make of these characters, this is brutal, uncompromising stuff, with a seriousness of purpose and a professionalism that do justice to the subject. The stage is sparse apart from an awful lot of trash – the play will rub our faces in the dirt of human nature until we see clearly through to the other side.