One minute you're sitting sipping a coffee, the next the dark has closed over your head and you're in horror land. It could happen that quickly, and it did in Chile, in Argentina and elsewhere in the 1970s – and no doubt is happening somewhere in the world even whilst you're reading this.

Thea Sharrock's all-female production of Spanish writer Fermin Cabal's journey into the heart of darkness, set in the period of Pinochet's military rule, is a virulent conscience-pricker. Backed by the Medical Foundation for the Care of Victims of Torture, it gets into your head and will most likely continue to quietly explode there long after particular details of this production have disappeared.

Tejas Verdes means Green Gables. But you won't find much in common with the winsome tale of the orphaned redhead on Canada's Prince Edward Island. Rather the horrific but simply told account of a deadly game of extermination, and the torture of Colorina (Shereen Martineau), whose main crime was to fall in love with the young and (big mistake) Marxist Miguel. Her other major fault, says one of the female doctors who attended her, was not to keep quiet but to insist on lodging a complaint about her treatment.

All of which is conveyed through a series of modulated monologues, framed as if in flashback as evidence given to an unseen judge, relating to the moment when Pinochet was arrested in London on extradition charges brought by the Spanish judge Baltasar Garzon.

The effect of Sharrock's cunningly intimate promenade production is to feel the collusion personally. It's as if you've stumbled into the actual graveyard where Colorina, and those like her, might have found their final resting place. When Martineau (the boyish Viola in the West End's recent Asian Twelfth Night, here beguilingly transformed) recounts what happened to her, the upbraiding is personal. There is no escape. She is right there beside us.

So, too, is Diana Hardcastle's haunted, eye-sunken friend, relaying her betrayal under torture, and Gemma Jones' gravedigger, unable to conceive after what she saw and was forced to do.

Not exactly a bundle of laughs, and Cabal's handling of the political context is, to say the least, sketchy. But Tejas Verdes is also shot through with moments of luminous beauty. And ultimately, for all its explosive material, it's that rare thing - even-handed.

- Carole Woddis