BoxedIn Theatre have created a splendid little venue in the courtyard of Dynamic Earth. Constructed from reclaimed materials, The Greenhouse’s USP is being the first zero waste venue at Edinburgh Fringe. Not only is this patchwork shed of a venue ethically sound, it’s an enticing space in which to watch a play. Seated in the round, with the wind creaking the walls and rafters, it feels unvarnished, even primitive, and very Fringe.

Swallows is a tricky introduction to the issues that the venue espouses, though. Tim, Harry and Ella are eco-warriors plotting to take world-saving action from within their hideaway in a post- or immediately pre-apocalyptic Earth. But that’s about the only thing that’s clear in a highly abstract play that puts atmosphere ahead of clarity.

Characters question each other constantly about their fears, or their nightmares, or what time it is. They squabble, they fight, they tease. They have identifiable characteristics – small boy Tim is enthusiastic but fearful; Harry is explosive and belligerent; Ella wary but capable of defiance. We never really learn who they are though, or why they’re there (except in the most general terms), or, for a while, even where they are. That bit only gets explained by a clunky exchange in which Harry describes the shipping container they’re in and its immediate surroundings. For a play which ostensibly has some further motive and a thriller-like tension to it, rather than a purely existential bent, such opaqueness doesn’t help. We only have the vaguest notion of what has drawn these characters into the conflict that plays out in front of us.

It’s a shame because the performances themselves are powerful and gripping, each actor fully absorbed in their role. Isabella Sheridan as Tim shows timidity and a burgeoning independent streak, Daniel Jonusas has tremendous presence as the cackling Harry, while Sarah Chamberlain exudes a certain wisdom. The finale is furious and potentially terrifying, the company making full use of the space and the structure that contains it to create something dark and powerful. If only we had a greater sense of what this climax represents.