What happens when romance turns into horror? Brad Birch slams two very different genres together in Black Mountain, and it turns out that even hearts can be haunted. Every relationship has its own ghosts.

Paul and Rebecca have gone on a retreat. They're staying in an empty old house in the middle of nowhere. There's nothing but stale coffee in the cupboards and Stephen King books on the shelves. They greet each other like strangers, awkward and over-polite, but they're not. They're lovers, just, making a last-ditch attempt to rescue their relationship. This isn't a holiday, says Rebecca. They came out here to talk.

Gradually, it becomes clear that Paul has been unfaithful. He wants to make it up to Rebecca, but she's hurt – so hurt that she'll struggle to forgive and forget; so hurt that she wants to hurt him right back. The security light flickers; damp in the fuse. Are they imagining it or is someone outside? Are the two of them alone?

Birch is having fun here, toying with gothic tropes and twisting them out of shape. There's an eerie old neighbour and an axe that goes AWOL. Showers start running by themselves, and creepy phone calls come in. It's all very knowing, but it pins down the idea that everyone has skeletons in their closet. Exes can hang in the air like spectres. The other woman can feel like a poltergeist. Jealousy can kill.

What Birch really gets right is the way relationships go awry. When the trust goes, it's little things needle and niggle until nothing feels right. Rebecca says she wants Paul to feel pain, to register the hurt he's caused her. That can't be why he keeps getting splinters, can it? There must be a reason the midges keep biting him and not her. You can feel the frustration between Hasan Dixon and Katie Elin-Salt, unnervingly implacable as Rebecca.

Birch gets the rhythms down pat – both of amorous irritation and of gothic horror. Short staccato scenes, bristling with frustration, take on an itchy, unpredictable energy, and James Grieve's production has a lot of fun with its spooks. Peter Small's lighting and Dominic Kennedy's sound combine like exposed nerves. It's a slight piece, almost too self-aware for its own good, but one with a dark heart.

Black Mountain runs at 13.25 at Paines Plough Roundabout @ Summerhall until 26 August.

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