Nature, so we are told, is red in tooth and claw. You can add "root" to that as far as Thomas Eccleshare's play Pastoral is concerned. When it begins we are in the kitchen of a flat in one of those soulless, inner-city tower blocks where Moll (Anna Calder-Marshall) is waiting to be collected. Most of her belongings have already been packed up; plants are beginning to sprout through the rickety floorboards.

Moll thinks she's going on holiday. What nobody has told her is that the whole town is being evacuated. Wild animals and aggressive vegetation are the new barbarian hordes battering at humanity's gates – and so far they're easily winning. Before long Moll, Manz (Hugh Skinner) and Hardy (Richard Riddell) find that they're trapped.

A young boy Arthur (Polly Frame) and his parents Mr and Mrs Plumb (Nigel Betts and Morag Siller) take what refuge they can as the trees and the animals continue their aggressive invasion. As allegories go, Eccleshare has given us a grim one, one in which the survival instinct in humans is shown to be as selfish and as ruthless in an adverse situation as though "civilisation" had never occurred to mankind.

The author and his director Steve Marmion are well served by both the designer (Michael Vale) – what Vale manages to achieve with a small stage is wondrous to behold – and the cast. Calder-Marshall is immensely moving as Moll, a woman who begins by being slightly batty but acquires a dignity of her own as other people disintegrate. Frame's study of an 11-year old boy who will now never realise his dreams is also excellent.

Bill Fellow is the conscientious delivery man whose fortuitous arrival marks the moment when the need to survive overwhelms basic humanity in everyone but the very old and the still young. Carrie Rock wanders in towards the end as the bride who has lost her hen night. As Moll and Arthur await their inevitable end, we yearn for some miracle to save them. Of course, we also know that this is never going to happen.