Judith Thompson has important things to say with her new play Watching Glory Die, which had its world premiere in Vancouver last week. It's a harrowing story, inspired by the life – and death – of Ashley Smith, the 19-year-old who choked herself to death in a jail cell at Grand Valley Institution in Ontario as guards watched on video monitors.

Like Smith, Thompson's Glory is first sent to a youth facility for throwing an apple at a postal worker, but this turns into a years-long incarceration that escalates from juvenile detention to prison, with stints in solitary confinement – or "therapeutic quiet." Anyone familiar with the tragic real-life case will be braced for the play's inevitable ending; Smith died in 2007, while under suicide watch.

Thompson, 59, is a celebrated Canadian playwright whose works include The Crackwalker, Lion in the Streets, Perfect Pie and Palace of the End. She has collected many awards and accolades over her more than 30-year career. In her first onstage role since the 1980s, she plays all three characters in Watching Glory Die: Glory, the Ashley Smith-based character; her mother Rosellen; and Gail, one of the prison guards on duty when Glory ends her life.

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The action begins while the audience is still being seated; Thompson, as Glory, shuffles onto the stage, stepping up into her jail cell – a two-sided cube with a mirrored floor. It takes some time for the audience chit-chat to die down as we slowly clue in to what is happening on the stage. Glory, clearly disturbed, focuses her attention on the jail cell walls.

The first character we hear from once the house lights go down is Gail – a plain-talking, working-class stiff who tries to play by the rules set by the corrections higher-ups, but who also brings some humanity to the enterprise. Gail wears an all-business blazer over her prison-issue blue dress, and black slip-on shoes. She tells a story about her brother, who was also a jail guard, and how it led to his demise. Message: Being a prison guard is hard.

Gail transforms into Rosellen – now in a ponytail, matronly cardigan and sandals – a devastated mother who can't understand how her child, adopted as a baby and raised with great love, has been incarcerated for five years. In one monologue with particular resonance, Rosellen tells the audience that she did her best as a parent – signed Glory up for dance lessons, watched Dora the Explorer with her – and yet this is what is has come to. Message: This could happen to your kid, too.

As Glory, Thompson, now shoeless, transforms into a teenager. At one point, in one of the play's finest moments, she shields herself mentally from a terrible prison episode by remembering a school trip, when a cute boy sat beside her on the bus and plopped one of her ear buds into his own ear, and they listened to her music together.

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There's a message here too – about the treatment of inmates in Canadian facilities, iatrogenic effects of incarceration and the justice system's inadequacies in handling mental illness.

A video camera projects Glory's image onto the cell wall to stunning effect – both visually and in a way that connects with Smith's terrible story. Also notable is a spectacular Taser event that gives the audience a visual jolt.

With these production values, this rich material, and what really is a fine performance by Thompson, one would expect a work of tremendous emotional heft. Instead, the work sags. The writing felt at times obvious. There is a palpable lack of tension. The transitions are clunky – changing footwear with each switch was awkward and distracting, and with Thompson fully in command of each of the characters, unnecessary.

On opening night, just as the performance was beginning, an earthquake struck off the coast of Vancouver Island. In the theatre, we felt nothing. For the next 80 minutes, watching what promised to be an emotional eruption on stage, I didn't feel much of anything either. With all this talent and possibility, that is a shame.

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Watching Glory Die continues in Vancouver until May 3, and then moves to Toronto, where it runs at the Berkeley Street Theatre Upstairs from May 15 to June 1.