Three casual workers arrive for the night shift at a meat factory. There’s hard-faced Becky (Victoria Moseley), timid, dowdy Susan (Kristin Hutchinson) and Grace (Janet Etuk), forced into employment despite her rheumatoid arthritis. They join Phil (Sean O’Callaghan), the only full-time member of staff and a gentle giant type – though like all of them, he seems bruised by life.

Their boss is Ian (Luke Clarke), a young guy with a bad goatee who considers himself a “very spiritual person” – although his life philosophy consists of masking his insecurities with a bravado drawn from The Apprentice and bad self-help books. He monstrously lords it over them, a mini-dictator in the small world of the factory, spouting jargon and refusing pleas for days off to visit children or for breaks in which to take medication.

Alexander Zeldin’s play premiered at East London’s The Yard theatre last year and it makes a welcome and timely return here: a stark look at the lives behind the political posturing on zero-hours contracts. Zeldin also directs, but the show was devised with the cast, and it shows – they all give terrifically subtle and affecting performances.

Harsh fluorescent lights remain up on the audience the whole time; the theatre is convincingly transformed into a grubby meat factory. Beyond Caring has a slow pace, its characters are taciturn – but they do actually clean. The audience must bear witness to a usually invisible industry; the repetitive scrubbing and sweeping and mopping and scouring, especially at the end when they literally bring in filthy hunks of heavy factory machinery, makes for uncomfortable viewing.

This show can be hard-going – but it’s a hell of a lot less hard watching than doing, we acknowledge guiltily to ourselves. Happily, Zeldin’s script is also very funny: understated but wincingly convincing in the manner of the best TV mockumentaries. The banality of the employees’ awkward attempts at social interaction, or the gapingly awful, petty-minded jobsworth comments that Ian comes out with, prompt frequent and slightly outraged laughter.

There may be a lot of silence and space in the production, but his writing has its own effective rhythm and, underneath it all, real compassion. The characters are all – with the exception of the perhaps over-vilified Ian – allowed moments of true tenderness as well as desperation.

Even so, at an hour and 45 minutes long, the play might have benefited from a trim; it can become wearing for an audience too. But it is surely right that presenting such worn-down individuals – spirits broken by an inhumane system of badly paid, back-to-back shifts with short breaks and harsh targets – should make you feel ashamed at what passes for economic sense in modern Britain.

To 16 May; nationaltheatre.org.uk