When Sweeney Todd lifts his razor high, it’s Eleanor Lovett who’s standing alongside him. The pie-shop owner is a central character in Sondheim’s musical, just as she is in the Victorian pulp fiction he drew on. But what depths of desperation or depravity could drive a woman to such crimes? Actor-playwright Lucy Roslin tackles that question in this eerily compelling work – which sees Eleanor welcome us to her basement kitchen, to talk us through the story of her life.

She’s mad, of course. Sane individuals, however ill-treated or marginalised, don’t turn other humans into pies. But there’s a seductive logic to Roslyn’s script: a disturbing memory from childhood, a promise bought and broken, a spark of infatuation at a moment of darkest need. It’s a story told through an eighteenth-century lens, where the church stands firmly at the heart of morality and Eleanor reaches for Biblical tales as readily as her own. And while none of this in isolation could explain what she does, pieced together, it’s uncannily convincing.

Piece it together you must. Eleanor shares her history in dismembered fragments, sometimes chasing digressions, often dropping into caricature impersonations of the other people she meets along the way. It takes some effort to follow – and just occasionally I felt the incursions from other characters drowned out Eleanor’s own voice – but the story’s enhanced by the disorder of its telling, admitting us little by little into Eleanor’s damaged mind and the real-life but distant-past version of London she lives in.

At first, Roslin plays Eleanor with studied casualness; a little too casual perhaps, on the verge of breaking out of character at times. There’s a horribly fascinating masterclass in butchery, a little forced jollity with the crowd. But as she recalls the wrongs of the past, the characterisation tightens: we see fragility and defiance, peace and devotion, all expressed with equal conviction through posture, face and tone. And though Eleanor spends most of the hour standing behind an intimidating butcher’s block, Jamie Firth’s direction is quietly dynamic, with repeated gestures effectively employed to link us back to earlier themes.

For the real story isn’t happening in this blood-stained cellar, but out in the clamour of London. The landscape of the tale is cleaved by the Thames, which splits the relatively salubrious Fleet Street from the sanctioned sin of Southwark; and there’s a fracture in society too, one that’s more complex than it seems and which Roslin’s script insightfully explores. By the end, when we suddenly realise which ritual she’s preparing for, it’s chillingly apparent how the framework of her existence could have led her here. A thoughtful prequel to a grotesque tale.