According to Wikipedia, Four Thieves Vinegar is a mix of vinegar and spices, believed in mediaeval times to offer protection from the plague. Matthias Richards, a 17th-century apothecary, has no truck for such superstitions; as the Great Plague of London begins to grip the city, he’s convinced his meticulous experiments have revealed a groundbreaking cure. There’s just one problem… his formula calls for gold. And right now, Matthias is locked up in a debtor’s prison.

It’s a clever choice of setting. In the midst of the plague, yet separate from it, Matthias and his cellmates are ideally placed to witness the horror unfolding around them. They can hear the bells toll to mark the dead - and from their window, they can see the uncannily deserted streets as the bustle of the city literally dies down. They hear reports, too, of the slow collapse of society, events which start with the flight of the King and eventually trickle out even to Newgate Gaol.

The first scene or two are perhaps a little slow, as we get to know Matthias and the women who share his cell. There’s petty thief Jennet, who’ll be hanged once her baby is born; and there’s the older, matronly Hannah, who lived through the last plague outbreak and is blessed with immunity now. Once the introductions are over though, Four Thieves Vinegar elegantly begins to unfold, revealing an engaging multi-layered plot. Nothing here is quite as it seems; all of the prisoners have secrets, and even their kind-hearted gaoler has a hidden ambition to play for.

Playwright Christine Foster grounds her story firmly in place and time, and the dialogue is peppered with informative allusions to the ways of 17th-century London. Just occasionally, she succumbs to nerdiness; the discussion of the legal quirk called the “neck verse”, for example, is an unnecessary complication. But far more often, the details lend both interest and authority, colouring in the backdrop of a city and society beyond the walls of the prison cell.

Liam Murray Scott delivers a rounded performance as the apothecary, at times detached and at times driven, single-minded in his mission and dangerously certain in his self-belief. Sorcha Brooks brings plenty of heart to the streetwise matron, whose sharp tongue and suspected larcenies conceal a far softer side; while Char Brockes also impresses as the condemned young woman, seeming at first a little girl lost but soon revealing something steely and, finally, tender. Special mention must go to Margot Jobbins, who stepped in at the last moment to play Holt the gaoler. Even though she was performing with script in hand, her crux scene lost none of its impact and poignancy.

Four Thieves Vinegar is a story well told, with fully-realised characters who jostle to out-smart each other even as their world implodes around them. It’s not a play “about” the plague; it’s a play about people, and about how people look after themselves when society fails in that task. There’s much to recommend this well-conceived and well-rounded production.