Lizzie Nunnery’s taut and terse two-hander defies black and white thinking. Like the best drama, it presents an ethical quandary that refuses to be boxed up as either right or wrong, with the implications of its central decision a matter of life and death nonetheless.

Nunnery’s focus is immigration and she makes you realise the complexities of a single case. Canaan (Wil Johnson) is from Zimbabwe, where he worked in Mugabe’s security force and as an MDC activist. He has been living in England for five years, but, having forgotten to resubmit for his status, he must go through the process from scratch under case owner Martha (Allyson Ava-Brown).

Like Joe Penhall’s Blue/Orange, The Swallowing Dark shows an individual tossed around by a bureaucratic system that cannot afford to admit his humanity. Canaan is a case to be judged, before he is a man – indeed a father – worth pitying. When Martha talks to him as such, it is off the record. If her dictaphone is whirring, she must stick to an official script.

The dilemma is between the right to refuge and the need to maintain its value. Cleverly, Nunnery makes you variously side both with and against a system that is over-cautious, inhumane and perfectly rational.

It seems clear that returning to Zimbabwe would leave Canaan in danger, but since the process must be legally watertight, the onus is on him to prove it beyond doubt. Martha, who gives her fifteen-year-old brother the benefit of the doubt against a manslaughter charge, is obliged not to offer the same kindness to Canaan.

Paul Robinson’s production makes the most of Nunnery’s knack for logical conversational courses, maintaining the back and forth ping, but opening up for Canaan’s carefully recounted stories. Alex Eales' intelligent design follows Canaan’s complaint of "having to justify myself every minute in this country," by making Britain seem one interrogation room after another.

However, the play’s underlying metaphor diminishes towards the end, which approaches television drama as it succumbs to ‘and then’ plotting. An ambiguous ending would have been more effective than the definite one Nunnery provides that lets sentiment seep in at the last.

Johnson and Ava-Brown give top-class, complex performances nonetheless. He leads us to presume Canaan’s innate goodness, but leaves cracks enough for her to sow seeds of doubt. Johnson treats every sentence tactically until passion or fear overwhelms self-censorship, while Ava-Brown shows the turmoil of the person behind the bureaucrat’s clipboard.

- Matt Trueman