Wind Bit Bitter, Bit Bit Bit Her is a show that wrong-foots you at every turn. It starts with that riddling title, which as well as being virtually incomprehensible, has almost nothing to do with what actually happens in the play. It carries on with the sudden appearance of a raging woman called Mary – who at first you might take for an abusive yob, before the reason for her distress becomes clear. And it continues unabated, on an epic scale, in a script that circles round horror and deep-state conspiracy… but in the end explores something more troubling still, the self-destructive spiral of a malfunctioning human mind.

I haven't the words to adequately praise actor Phoebe Vigor, whose restless physicality and disturbing intensity make Mary's turmoil understandable and real. The story's narrated by Mary, told from inside her head, as she suffers a life-changing trauma and endures the isolation of grief. There's tragedy, then loneliness, then something I wasn't expecting. And Sami Ibrahim's clever script leads us confidently through these phases – darting dizzyingly back and forth between reality and fantasy, but never quite leaving us behind.

The staging's stark and simple. Rectangles of light set out a runway for Vigor to stalk up and down; with the audience seated facing each other, visible to each other and to the actor, there's nowhere for anyone to hide. The uncompromising performance and sharply-honed script draw us convincingly into Mary's collapsing world, showing how a single nudge at just the wrong moment has the power to destroy a life from within. And a big surprise in the middle subverts your expectations of the play's own structure and form, offering a welcome change of pace when the intensity threatens to grow overwhelming.

But it's all just a bit too long. An extended phase of gothic horror is cleverly worked, gently hinting that what we're hearing is unreliable – a twisted interpretation of a relatively unremarkable scene. But once we've grasped the key point – that Mary is descending into a troubled fantasy – the sense of peril ebbs away, leaving us with an overblown grotesque drama which goes on far too long. Similarly, the extended voice-over which ends the piece adds no new insight into Mary's mental state, and carries on for several minutes after I thought the play had reached its natural end.

A good edit could bring this down from 80 minutes to the regulation Fringe hour, which I'd say is as long as a work this full-on really needs to be. But the earlier scenes, before Mary spins away from us, are among the most powerful you'll ever see – and the depiction of grief driving a loving couple apart is both searing and utterly believable. Wind Bit Bitter… is a bold play, risky and exposing for both actor and playwright, and on the whole it pays off well. This is the kind of work the Fringe is for: daring, thought-provoking, and emotionally raw.