One of the great joys of theatre is the contract of imagination between performer and spectator: if the former can conjure up another world, the latter will enter it with them. A bonus, then, to have not one, but layers upon layers of worlds evoked by the two female cellmates in Alice Birch's Little on the Inside, from their dismal reality and fractured past to their dreamlike shared fantasies. However, what for us is a quick sojourn into another realm is for them a vital means of survival, and its loss could spell devastation.

This compelling 45-minute snapshot was written as a response to another Clean Break production, Rebecca Prichard's Dream Pill, and both find effectively theatrical ways of framing harrowing, urgent issues. Birch avoids both melodramatic cliché and solemn preaching by focusing on the women's perfectly symbiotic relationship and their sly glee in narrating and reframing events, conducting and directing one another with easy rapport.

The energy and wit of their re-enactment prevents the evening from becoming unbearably grim, as violent, traumatic experiences are related with bracing stoicism and a captivating eagerness to entertain. Simone Jones's B has the cheeky swagger of the natural raconteur, brilliantly impersonating others and storming through story as she builds up to a punch line, while Susan Wokoma's A balances lyrical evocation and wry observation with quiet gravity.

Underpinning their recitation is a surprisingly tender relationship, forged in mutual determination to break out of the enforced role of victim, but there are also interesting notes of ambiguity as they teasingly compete with and contradict one another; A in particular understands the fine line between fiction and deceit. The two women have the dangerous intimacy of family members or old lovers, using their extensive knowledge both for fierce support and the occasional well-aimed barb.

The strongest moments of Lucy Morrison's taut production come during the pair's fast-paced interactions, trading memories and exchanging refrains to create a dynamic, rhythmic soundscape; there are times when their harmonising verges on spoken-word poetry. The necessity of narrative as a means of escape gives their performance a compelling urgency, as B comes to realise that her gift for storytelling is a powerful currency in this environment and A tries to outrun the horror of silence.

Some of the overtly emotional beats don't land quite as effectively, particularly when Birch shifts tone abruptly or begins to labour the point, most notably in the bumpy climax. Arguably, the theatrical space doesn't offer much in the way of amelioration, as the Almeida's airy foyer, with its expansive skylights and ambient hum from the bar, creates an extra challenge for the performers by detracting from the supposed claustrophobia of their surroundings.

In future, the play might be better off staged in the round to give us better contact with the performers, and the design is a little confused; why, when the piece celebrates the power of words to paint Elysian visuals, do we need a vaguely turf-like carpet and screensaver sunset backdrop?

However, minor quibbles aside, Birch's short piece packs a real punch by committing to a rich portrayal of female friendship, without shying away from the reality that co-dependence is a risk in prison; if another person is almost literally your whole world, what happens when you face separation? It is to Birch's credit that we vividly understand how such losses are terrifyingly magnified.

Little on the Inside, at Almeida TheatreMarianka Swain reviews Little on the Inside at the Almeida Festival.4