I should have expected Cargo to be harrowing.

In fact, if I’d thought about it I should have arranged an intravenous drip to rehydrate me after all the weeping, and a night-light to keep the shadows away.

It’s not that Cargo is grotesque or egregious in any way. It’s balanced, delicate, expertly-written. It’s simply that this is a play about migrants trying to make it to Europe.

There are child refugees with terrifying tales of trauma, desperate men with shifting identities, shaking women with haunted pasts. If that wasn’t enough of a clue, it’s right there in the title: humans turned into cargo.

Even so, Cargo packed a particularly powerful punch to the gut. And it’s entirely worth it.

It starts with a familiar tale. We are crouched in the hold of a cargo ship. Between the crates, a boy, girl and a woman try to stay quiet enough, invisible enough, to make it across the seas. From the darkness comes a flicker of light, and then a spark of conversation. And the dialogue races from here: realist, well-judged and perfectly timed.

Tess Berry-Hart is a magnificent writer: her characters speak easy small-talk peppered with fear and mistrust. It’s exactly how we should speak in a world where children cross oceans alone. In other words, it feels horrifying and humdrum at the same time.

Cargo looks the part, too. The audience sits on packing cases just as Joey (Milly Thomas), Ismael (Jack Gouldbourne) and Sarah (Debbie Korley) do. The wail of foghorns, footsteps above and around us hold us in a small dark box of fear. We can’t experience the depth of their vulnerability, but we sense something of it.

The studio setting works well in several ways. Small movement are magnified, so this tiny space never gets boring. It’s flexible – becoming in turn menacing, intimate and claustrophobic. Lastly, it establishes a rigid divide between ‘inside’ and ‘outside’. As a metaphor for the clefts between people, it’s perfect. We have ‘fortress Europe’ on the outside, and the migrants huddled together within. There’s the sense that, in Europe, these people will be seen as ‘all the same’. Within the hold, of course, they carefully size up one another’s allegiances and origins. As Kayffe (John Shwab) jokes, they may be ‘all…in the same boat’ but that’s where the unity ends.

Through guarded exchanges we learn about rebel forces, up in the mountains, about the bombings and executions. So far, so Syria.

But as the ship rolls onwards through the dark, the world around us grows murkier too. A new arrival bursts into the scene, and our sense of certainty – cobweb-thin as it was – begins to give way.

Somewhere across the channel, we totally lose our moorings. Not so much a plot twist as a plot crunch, Berry-Hart finally cuts all the lines. Spinning in the ether, Cargo takes on a darker and more dystopian tone.

It’s a gripping 80 minutes (as you’d hope, when you’re sitting on packing crates and bin liners). Although it’s snappy, Cargo has something more lingering to offer. When boy refugee Ismail asks Sarah why ‘they’ (in Europe) hate ‘us’, her answer is powerful: ‘they remind us’ of how ‘[fragile] their world is’. Their life of lattes and coupés can give way all too easily.

Few of us will make a journey over the seas. But we should all take this journey with Ismail, Joey, Sarah and Kayffe.

Cargo is playing The Arcola until 6 August. For more information and tickets, see The Arcola Theatre website.

Photo: Mark Douet