As we enter a silent theatre, we're invited to take a letter: a letter to read, reflect on, and share with the others around us. These are no ordinary letters, but victim impact statements, evidence submitted to a court to explain how a crime has affected the people it's left behind. Some of the writing is measured, some of it is raw; but all of it is heartfelt, speaking of deep trauma and inconsolable loss. There's been a murder, it seems. Perhaps more than one.

So it's a surprise when the black drapes part, and an ordinary man – a likeable man – walks onto the stage. He's the murderer the letters are talking about, and he's here to tell his story; a well-worked, fully-fleshed tale, which starts with a chance meeting in a railway carriage and ends with an impact of an all-too-literal kind. Along the way we hear of politics and conspiracies, of puppets and puppet-masters, and of sincere convictions bent out of shape – consumed by personal ambition and purposeless malice.

In one sense, the man's story seems outlandish and extreme, but in another it's disturbingly of our time. He describes a form of radicalisation, but one which has nothing to do with culture or religion – a clever transposition from playwright Jonathon Carr which makes the man's motivation that little bit easier to connect with and understand. But there's something else going on too, something even darker: a kind of sociopathy born of complete boredom, the ultimate product of a soulless society which has somehow left our narrator behind.

There's an uncanny distance in Richard Henderson's portrayal of the man, a sense that this explanation for what he's done has been rehearsed and regurgitated many times before. If we're hearing a confession, it's a callous one; the performance – and the script – is more about coldness than it is about feeling. He delights in explaining how he moulded others, how he manipulated and then overpowered them, and the sudden realisation of exactly what crime he's committed leads the way to a profoundly disquieting ending.

Which brings us back to those letters, which we read at the start of the show. They certainly add a heavy weight to the story – foreshadowing events which, at first, seem impossibly distant from the mundane world described. But I'd expected to feel more inner conflict and psychological tension; a gut-level response to reading about a murder, and then seeing the murderer in front of us on the stage. Perhaps it's just that the written word is inherently less immediate than an actor in the flesh, but I found that the letters flitted quickly from my mind as soon as the play proper began.

For me then, the show's defining gambit didn't add all that much. But it didn't take anything away either – and this monologue is powerful and troubling enough to stand up without any gimmicks to support it. Carefully crafted and disturbingly performed, Impact is both an insightful warning, and a tense, intriguing, intelligent tale.