Although the feeling is suggested in many plays, it is rare that one truly feels like a fly-on-the-wall when at the theatre. BAFTA-winning writer Jack Thorne’s new play takes its name from a family of short living stinging flies, and there are times when one truly does feel like an unwelcome and unnoticed observer in a rather intimate place.

Mydidae is set entirely in married couple Marian and David’s bathroom and takes place over the course of a single day, from the banter of morning ablutions to a disastrously failed attempt at a romantic candle-lit bath in the evening. Their relationship initially seems effortlessly comfortable; there are no qualms about peeing or flossing or shaving in front of one another. Gradually, though, Thorne’s script reveals that this day is the painful anniversary of a shattering event that has left its mark on both characters and an indelible scar on their relationship. They are held together as much by despair and remorse as by the love they once shared.

At times it seems unlikely that two characters who have, in some respects, so much emotional distance between them would spend quite so much time being as intimate as these two do. However, despite a slightly stilted opening which feels scripted more along the lines of observational comedy than naturalism, there are moments of cutting truth and even startling beauty in the production.

Vicky Jones directs the production with a light hand, aiming for low-key naturalism that is, on the whole, highly effective. What is most noticeable is the beauty that has been found in stillness; the bath becomes a world in itself, which both confines and releases the inhabitants. Amy Jane Cook’s set does well to transform the sometimes-troublesome upstairs space at the Soho Theatre into a fully-plumbed bathroom, which is simple in its accuracy and detailed in its precision.

Jack Thorne’s script has a dark sense of humour throughout and, although it doesn’t always avoid feeling a little contrived, finds a great deal of truth. The drip-feeding of information is at once intriguing and irksome, as it doesn’t allow the audience to really get into the characters’ heads. This results in what occasionally feels like a slightly superficial impression of them. However, Thorne uses the location movingly; bathrooms are where we are at our most exposed, our most rudimentary, our most bare. The characters are, by the climax, both physically and emotionally naked. There is something striking in this simplicity. Thorne also has a natural ability to create sharp-edged dialogue that zips along with pace and vivacity.

This is a beautifully acted piece. Phoebe Waller-Bridge finds the pain and vulnerability beneath Marian’s kooky, gawky exterior. She has a wonderful presence and gives a very sincere, touching performance. Keir Charles’ performance as David is also painstakingly executed, with insecurity, longing and humour all balanced beneath his sense of guilt. Charles finds a desperation in David that stimulates his actions through the play, and this is beautifully painful to watch.

Thorne’s play is a thoughtful examination of hope despite hopelessness, of relationships under unimaginable strain. The power in the script is, often, in the things that are left unsaid. It is shocking and surprising in a way that is rare in the theatre and the sense of uncomfortable voyeurism created is rather absorbing. Despite a slow start and some difficulties in uniting the themes and the setting, this is a captivating piece of theatre that leaves an audience with much to think about.

Mydidae is playing at the Soho Theatre until 22 December. Visit the Soho Theatre website for more information and to book tickets. Photo by Simon Annand.