Do you like mustard? The sting of its heat, the taste that borders on pain, that rush of sensation as the vapours hit your nose? This piquant solo play, from the much-admired Eva O’Connor, might make you think again: a study in mental vulnerability and defiant survival, it uses the fiery condiment as more than just a metaphor. It’s humorous and impish – a delight to see performed – but at times it’s also heart-rending.

Mustard is the story of a woman, known only as E, who’s moved from rural Ireland to Elephant and Castle. One night she meets a handsome broad-shouldered man, who carries her off to his mansion in leafy London suburbia. But E is projecting a fairytale onto a slightly-less-perfect reality – and once the passion cools, her lover’s charm turns bitter. It’s then that E’s latent fragility bursts through and threatens to overpower her.

The joy of O’Connor’s self-penned script lies in E’s mischievous narration. It’s sardonic and unfiltered, but there’s no hint of malice; and it has eloquence and poetry to complement its lively humour. You’ll need to concentrate, because the words flit by in a series of verbal images, every one of them fleeting yet worth catching onto. The actual stage in front of us is cavernous and spartan – yet E’s world, and the characters in it, feels intimate and real.

But as we get to know E better, we realise that something is wrong… and that wrenchingly, scarily, she knows it too. She uses mustard – yes, literal mustard – in a form of self-harm; she’s acutely aware of how this damages her, and of the pain it causes her mother. As the fairytale unravels, the images grow darker, and E wonders aloud if there’s any escape from the madness she feels engulf her. The self-awareness is haunting – as is E’s conscious surrender to a fixation she knows will destroy her.

There’s a surprising aspect to the second half of the production, which borders almost on performance art. I won’t spoil what happens, but it’s bold, and it’s sure to split opinion. It distances us from the character that O’Connor has worked so hard to build – but maybe that’s the point? I also won’t spoil the ending, but I will say that it’s an unexpected turnaround, and I’d have liked to understand a little better how it came to pass.

In the end though, Mustard is all about things that are hard to understand, even for those who experience them. E’s difficulties may manifest themselves in a very specific way, but O’Connor’s script clearly speaks for a far wider cross-section of society – and it’s rare for a show which tackles such a weighty topic to feel so light on its feet. Like the condiment itself, then, Mustard delivers a hit of both exhilarating pleasure and bitter pain.