It's very fitting that James Craze opens this show about his grandfather, Ernie, with the admission that this is just the story of a man whose life was in no way special - a very ordinary life. For this isn't a story of heroism or villainy, of grand schemes and plans nor great romance: it's just the very normal story of a man and a life lived.

Unlike the more romanticised (and, of course, fictional) works that tell the story of a single life (such as William Boyd's Any Human Heart), it's Ernie's sheer banality that makes it so engaging. We're not seeing a man inserted into any situations that show him to be anything more than a normal man – we're seeing the simple life of one who reacted to situations in a way you can empathise with and understand, which is probably one of the elements that make this so touching to watch.

There's also a slightly more tragic tale here: the fact that Ernie Holt wrote an autobiography and failed to get it published. Perhaps his story was just too ordinary, perhaps the writing wasn't up to scratch, perhaps it wasn't to the publisher's taste – there's any number of reasons why this tale wasn't published, but James Craze has clearly taken that to heart. There's a bold, unashamed personal mission in his retelling that speaks of a high respect and love for this figure, and it's a testament to his ability that what could have been impenetrable and introspective is so watchable.

For Craze plays them all – Ernie himself, and every other character he meets in his life. The performance is a physical theatre masterclass, with Craze leaping into different people's skin with subtle (and some not so subtle) twists and alterations and embodying many a situation that would otherwise have been rather hard to stage. But Craze does it all – he takes us from parade grounds to boiler rooms and fills them with interesting characters, all the while maintaining the central line that is Ernie's wry and warm take on events, all on a blank stage with only some small costume changes. It's a very impressive display.

All the more impressively, he achieves this with the requisite whirl of energy needed to populate the stage with just himself. He leaps from scene to scene at a relentless pace, perfectly balanced at pacey without slipping into helter-skelter. However, this is where some of the cracks start to emerge: a few of the quieter moments hint at greater "issues" – poverty, slavery and race relations are all referenced, but avoided, and this stands out too much to not be intentional. These either need to be discarded or expanded upon – at the moment they break the pace and pull focus.

There's also a slightly thornier issue that the piece suffers from, but it may not be one that's solveable: once Ernie has survived World War II and left the Navy, the pace and noteable events slow dramatically – it's unclear whether this is down to Craze's interpretation, Ernie's original autobiography or just the fact that less happened to him after the war, but this is a dramatic hole. But, with Craze telling his grandfather's story, it's not one that can really be fixed: life is life, and this is a story of what happened, not what works well as a plot.

However, this unevenness doesn't drag too much: in the end, the piece is so short that it doesn't matter too much. It could easily be lengthened, and I'd suggest more time on Ernie's post-war life and animating it more fully, but the emotional ending is theatrically simple, emotionally arresting and a wonderful image – bravo!

Ernie, at Theatre503Chris Hislop reviews Ernie at Theatre503.4