Brimstone and Treacle is arguably not so controversial now as when it was originally written for TV in 1976 (but still wasn’t screened until ten years later as the Director of Programmes thought it was “nauseating”). The rape of a vulnerable girl and unabashed racism are shocking, but above all it is the questionable moral compass that disturbs in Brimstone and Treacle.

The characters are simple, flawed human beings, but Potter forces a distance between the audience and play so one can’t help but sit up – not back – and question the origin of the inexplicable chaos existing in our world. The play follows the Bates, whose marriage has been pushed to the edge by their daughter, Pattie, being left mute and incapacitated from a hit-and-run accident. Things take a turn for the strange when man called Martin arrives returning Mr Bates’s lost wallet and proclaims (poetically and dramatically) that he once loved Pattie, and would like to help care for her.

The unbearable silence which opens Brimstone and Treacle makes your skin prickle, but this tension comes and goes temperamentally. Martin (played slyly by soft-spoken Rupert Friend) speaks with his eyes, shooting glances at the audience so that we know something’s wrong. This dramatic irony stimulates fear, but also reminds the audience that every word Martin says is a lie. The consequently humourous reaction to his treacle-sweet façade detracts from the impact the play could have – however magical Potter’s language and intimate the Arcola may be.

Still, Friend’s Martin is a wonderful enigma, a devil with the assumed appearance of an angel. Mr Bates speculates that their stranger could be the “devil himself” which appears to be spot on. He is the symbol at the centre of Dennis Potter’s exploration of good and evil, and the reality that the world is not necessarily as logical as faith or political structures would imply. Bad actions bring about good consequences and vice versa, proposing the Machiavellian question: does the end justify the means?  Martin asks, if Mr Bates wants all the ‘blacks’ to leave England, but they won’t, then is a concentration camp justified? [SPOILER ALERT] This controversial notion reaches its height in the scene where by raping Pattie, Martin miraculously heals her, disturbing the audience’s pre-conceived mores and making Brimstone and Treacle a truly haunting play.

Mr and Mrs Bates are also the antithesis of one another: Mrs Bates is religious, open and eager to trust Martin, and Mr Bates is most definitely not. Both Ian Redford and Tessa Peake-Jones inhabit their characters with a necessary sense of truth, complemented by Alex Eales’s authentic and reserved set. The Bates trust Martin because they equivocate goodness with his outward appearance: a conservative in Thatcher’s Britain, a gentleman – even a “saviour” to Mrs Bates as he relieves her of her burdens. This couple prize “the England I used to know”, an ideal which can no longer exist for them, and so has shattered in parallel with their daughter’s quality of life. Matti Houghton is transformed as Pattie Bates; not once in the 90 minutes does her physical intensity waver, or tormented eyes lose their vivacity. Punk rock music punctuates Houghton’s wild movements in the explosive segments between scenes, where Pattie relives the car crash, and her internal anguish can only find a physical outlet. It is both a subtle and startling performance.

Director Amelia Sears takes no prisoners in this challenging play. Brimstone and treacle was a Victorian concoction believed to do you good, and although Sears’s production might leave a sour taste in your mouth, it will definitely enlighten you.

Brimstone and Treacle is playing at the Arcola until 2 June. For tickets and more information, go to the Arcola website.