I know straight away that something special is on its way: on entry to the lecture theatre which forms the space for this show, the smoke machine and coloured lights are on, and Cherie Moore is already singing. Moore has one helluva voice, its power invoking both goosebumps and awe as I find a seat. I’m a bit confused to start with: I came to see a dramatic exploration of the way mental illness has affected a family, and only later noted that I’d missed the reference to cabaret in the show blurb. As it turns out, it's described elsewhere as gig-theatre, intended to bring the work to audiences who love storytelling and music but wouldn’t naturally incline to the theatre.

We meet musicians Tom Broome on drums and Robin Kelly on keyboards. The Valerie of the title is Kelly’s grandmother; the show is an expression of his gratitude to her, and strong women in every community who act as a family’s bulwark.

It’s also Kelly’s own enquiry into what he might have inherited genetically. Kelly’s mother’s family tree – marked out on Broome’s back – shows almost everyone suffering from mental illness, life in one case ending in suicide. In his own take on the nature-or-nurture debate, Kelly has done some research into DNA, which he shares under the spotlight. Genetics might be rocket science to me, but Kelly’s quest is clear: is mental illness in the DNA – his DNA?

Valerie’s story is challenging, uncomfortable to share. Kelly says that when he asked her permission to make a piece of work to portray her life, she replied that she didn’t think there was much to say. Yet Kelly has so much to thank her for, not least for keeping his mother safe as a child, living with a man with a complex soul who had lost reason to violence.

Powerful numbers written by Kelly and Moore are interspersed with excerpts from Valerie’s life, rendered mainly by Moore with a little help from her friends. Broome reads out excerpts from Valerie’s letters, rewritten on long sheets of paper. Most moving and provocative is the repeated reference to Valerie’s marriage vows – when an unpredictable life brings danger, she reminds herself that she took on her man ‘for better or for worse’. Ultimately she decides she must choose safety for her children, but arriving at that choice was not straightforward for this woman, loyal to her vows and her husband.

Kelly ends the show with a raw exposé of his own vulnerability – the fears which keep him awake at night, and symptoms which worry him. Symptoms like sitting in a crowd of people, and feeling alone as blood rushes to head, as the noise drowns out voices around so he feels like he can only lip read. What is going to happen is a question which cannot now be answered, and underlies his constant worry. Will advances in medical science even help?

Last Tapes bring intimate, moving and challenging works to the stage, and ask their audiences to engage in important conversations about social issues – we are invited to join them in the bar afterwards. Valerie is an extraordinary show in the story it tells, the issue it raises, and the overwhelming sense of truth exposed, which fills the theatre as powerfully as Moore’s captivating voice. My only complaint is: there are no CDs to buy at the end of the show.