You know a show is something special when you’re disappointed to reach the interval.

As the lights came up my companion and I were in tears, and judging by the snuffles progressing around the dress circle through the second act, a significant portion of the audience were crying by the end, bowled over by a production of extraordinary emotional depth. Director Sally Cookson has a track record in adapting powerful stories of adolescence for the stage: La Strada (Belgrade Theatre Coventry) and Hetty Feather (West End), to name a couple, feature a young protagonist’s search for meaning in life. But neither of these provoked anything near the audience reaction to A Monster Calls’ fearless exploration of illness, loss and grief.

Devised by the cast, director, and ‘writer in the room’ Adam Peck, A Monster Calls is an adaptation of Patrick Ness’ bestselling novel. It is surprisingly the first collaboration between Bristol Old Vic and its sister company, The Old Vic in London, since Bristol Old Vic was established in 1946, and demonstrates the imaginative potential in combining regional and London theatres. This production defies the (arguably outdated) conception of British theatre as either a product of London or ‘regional’ – Cookson was born in London but lives in Bristol and is an associate artist at Bristol Old Vic. Having been devised for both venues, the show is not tied to one particular place, so feels accessible in being understated. Cookson says of the book: “It’s marketed as a young adult novel but it’s really a book for everyone,” and the same can be said of her adaptation.

Conor O’Malley’s mum has cancer. But even with an absent dad, meddling grandmother and a gang of bullies to deal with at school, he manages to support her through treatment and carry on as normal; until he starts being woken at night by dreams of swirling blood and screaming. A monster is calling to him from the darkness.

“I will tell you three stories,” it growls, “and then you will tell me a fourth.

You will tell me the truth. Your truth, Conor O’Malley.”

As Conor watches, it seems to grow out of the shadows around the stage, into the unsettling form of a yew tree with a human face. Stuart Goodwin as the monster is simultaneously menacing and warm, his shape-shifting tales undermining what Conor thinks he knows about life. His unearthly nature is evoked through music: with influences of the darker side of folk ballad traditions, synthesisers and loop pedals bring an other-worldly, contemporary quality. Composer Benji Bower developed much of the score in the rehearsal room, improvising alongside the actors, and the cast are integral to the music. From their positions in the air and on all sides of the stage, the cast sing live and their vocals are looped and repeated back around the auditorium, bringing the monster’s story scenes to life.

“Your mind will contradict itself,” the monster warns Conor and Cookson’s production echoes this contradiction, blending a naturalistic soundscape of school yards and hospital rooms with ensemble-built physical theatre. A tangle of limbs and ropes form tree roots; a row of actors on chairs become classrooms, clocks, kitchen cabinets, appearing to melt in and out of walls and furniture. In this space where shapes and stories run wild, it’s difficult to know what you can believe in, but it’s this blurring of reality’s edges that eventually allows Conor, and the audience, to face his agonising truth.

While the music is spectacular, the production’s style is unassuming, with none of the glamour or showiness of other physical theatre productions (Metta Theatre springs to mind). In their grey, loose-fitting clothes the cast could almost be in the rehearsal room.

In fact, A Monster Calls will undergo a further week of rehearsal and development, as part of Cookson’s iterative approach to theatre-making, before beginning a six-week run at the Old Vic in London on 7 July. The show will benefit from the opportunity to iron out occasional hitches in transitions between scenes where the ensemble might move more fluidly together.

That said, the interplay between Matthew Tennyson (Conor) and the rest of the ensemble is difficult to fault – especially in the high school scenes. Moments when the ensemble freeze and twist around in their chairs to stare at Conor convey the acute, painful isolation that comes with having something bigger than the every day to worry about. As he moves in slow motion under their collective gaze, looking bleakly ahead at the audience, it’s as if he exists in a parallel universe to his classmates – he can interact with them but their realities are completely different.

Like the show as a whole, Matthew Tennyson is sensitive but never sentimental, and entirely convincing as thirteen-year-old Conor. He captures the limbo of adolescence: hovering between occasional glimpses of childish goofiness and a fierce need to be taken seriously as an adult. With his mother’s illness uprooting Conor’s life, the building tension in Tennyson’s movement conveys the weight of emotions he can’t express, until the full force of his anguish and guilt explode in the show’s soul-bearing final scenes.

The simplicity of the staging belies the production’s power and weight, or perhaps makes space for it. The set, such as it is, comprises two rows of chairs, several ropes hanging down onto the stage, and a huge digital screen that functions as backdrop, projecting images of Conor’s bloody nightmares or the forests in the monster’s stories. Musicians Benji and Will Bower are on display in a box set into the screen above the stage. The whole show exposes its workings, proclaiming its anti-realism starkly from the beginning.

Yet this laying open of theatrical artifice paradoxically makes the show feel human – it reminds us we are all performing every day, trying to navigate through trauma and joys, and often getting it wrong. Conor’s brisk, keeping-up-appearances Grandma (Selina Cadell) will later howl uncontrollably in rage and pain; his Dad (Felix Hayes) attempts to shield him from worry while simultaneously looking to him to help cope with his own distress. Like the monster and his shifting, unpredictable stories, life is rarely as straightforward as good guys and bad guys.

Set Designer Michael Vale says:

“The design shies away from set pieces and complicated scene changes… Sally and I both decided early on that the design should be simple and clear, allowing the audience to augment the images with their own imagination in much the same way as a book does.”

The result is a uniquely absorbing show that exists beyond the stage: production images don’t do it justice because so much of it is subject to the individual audience member’s imagination. The monster himself is a cleverly created outline, built by the ensemble, ropes and lighting around Stuart Goodwin’s powerful form – a sketch that leaves space for our own minds to fill in the blanks. By limiting what is shown, the production prompts imagination to layer our personal ‘monster’ onto the tendrils of rope: picturing what might be straining sinews, or perhaps looming red eyes. Through playing this part in shaping the monster, each audience member projects the fears – and hopes (the monster treads a fine line between guardian and predator) – of their own story.

It shouldn’t work, but it does – perhaps it’s necessary even, to approach a narrative about grief and the mind in this bare, non-patronising style. Cookson and her cast trust their audience not to need hand-holding, but to brave the journey with them, to face the truths this play is exploring and feel them.

Marianne Oldham (Mum) especially stands out for her raw, heart-wrenching final scene with Conor. Her physicality conveys both fragility and strength; in counterpoint to Conor’s tense, hardened posture she stretches forward in an echo of the monster’s ropes as she attempts to bridge the distance that has grown between them. In a shift from defensively underplaying the effects of her treatment, she summons all her energy to give her son what many of us are afraid to give ourselves – permission to feel.

Initial reviewers have dubbed A Monster Calls ‘astonishing’ and they are not wrong, but no amount of words can convey what it feels like to be part of a story that pushes the limits of people and theatre. You need to witness it.