As the lights come up on this original piece of youth theatre, we find three young people – two brothers and a sister – gathered at an open grave. Their parents, we learn, are missing; the older brother, just 18, is bringing up the family alone. But it's not mum or dad in the coffin. So just who – or what – are they burying?

The play's called Life Without Dog, which you might think is a spoiler, but the answer's not as simple as it seems. The children's relationship with the eponymous Dog is a carefully-worked enigma; it's evident they've lost not just a beloved pet, but a trusted confidant and friend. And the three have a lot to confide about: abandoned by their parents, they face the challenges of the adult world alongside the confusion of their teenage years, all while squabbling and supporting each other as brothers and sisters do.

Dog appears in flashbacks, as an adorably ragged puppet – and there's a particularly neat gambit involving puppeteer Laurie Cousins, which adds a surprising new dimension to the performance style just as things were starting to flag. The resolution of the central mystery isn't quite as clear as I think they intended it to be, and even after the end of the play it took a bit of head-scratching to work out exactly what we'd seen. But it's worth the mental exercise, because the threads of plot come pleasingly together – and a few puzzling details which I'd written off as continuity errors turn out, in fact, to be very much part of the plan.

So the concept and plot are clever, but they're let down a touch by some unnaturally stilted dialogue. The basic idea – that the kids share thoughts with Dog which they daren't discuss with other – is certainly a smart one. But those secret thoughts come out in structured, well-argued sentences, like words read from a book rather than halting confessions from within. If I gave one piece of advice to these talented young creatives, it would be to focus on the way that people speak in real life – and particularly on how speech changes as emotions kick in.

Ben Kirkwood (as middle sibling Maxi) has many of the script's funniest lines, and sells them well; as the most overtly troubled of the characters, he's convincing in more serious moments, too. Emily Kirkwood as the younger sister portrays a delightful impishness, both through an ever-expressive face and occasional moments of energising physical comedy. Sean Kilty plays the oldest brother and, just like his character, carries the burden of holding things together. He has the command and presence his role demands.

Life Without Dog works on many levels: as a study in loss, as a clever conceit, and as a reminder that although we can be cruel to those we love we can trust each other to pull together in the end. With plenty of humour and some moments of genuine poignancy, I hope and expect it'll prove a springboard to future artistic careers.