Threeway[author-post-rating] (4/5 stars) There can’t be many shows at this year’s Fringe with a madder premise than Threeway. What opens as a naturalistic comedy of manners and sexual adventure soon becomes something far stranger – and very much funnier.

Fringe First winner DC Jackson has an ear for realistic dialogue and the couple at the heart of this piece ring true throughout. Julie and Andrew have been together for some time and have a level of comfort and ease in each other’s company that is strangely romantic, even while they are discussing their plan to become “pan-sexual adventurers”. They have decided to branch out, try something new; they’ve found a stranger on the internet and he’s coming over.

When Mark arrives at their house for the pre-arranged threesome they have ordered from the internet like a package from Amazon, everything is awkward in the way you would expect. But when the three of them wake up together the next morning, it soon becomes evident that things have gone very wrong indeed. Andrew is now inside Julie’s body; Julie is in Mark’s; Mark is in Andrew’s. Everybody gives up on whatever they had planned for that morning to spend a little bit of time screaming and swearing.

There have been plenty of body-swap films over the years – Freaky Friday was remade only a decade ago – but Threeway is a true modernisation, a revolutionising of the trope, because Jackson has succeeded in finding the one element that could make it even more impossible to explain.

If it sounds gimmicky, it somehow isn’t. Having a stranger in their house who can’t really leave adds an extra layer of tension to a relationship that is clearly more complicated than it looks at first glance, not least because the stranger is sitting in Andrew’s body as well as his living room. Jackson actually uses the heightened drama of the situation to conduct a thorough, ice-cold unravelling of a very believable relationship.

The things they all inevitably learn about each other’s perspectives are also rather more nuanced than simply that other people have lives as real as you own. Julie, for instance, is horrified to receive a racist slur when standing in the street in Mark’s body, having believed Glasgow to be far more permissive than it apparently is. Similarly, Andrew, who has the horrifying task of going to work as his girlfriend, a nurse, is appalled to realise how much casually inappropriate sexual ‘banter’ is directed at her every day. Of course Julie’s never mentioned it before. Why would she? It isn’t remarkable; unlike Mark, she’s been a woman all her life.

All of the characters are wonderfully developed and realistic for something so high-concept, especially Julie, who manages to be a sexually adventurous woman without that defining her entire character. Gabriel Quigley shines as both Julie and her long-term boyfriend; Joe Dixon is excellent as Julie and Mark both; Brian Ferguson brings plenty of humour to Mark and pathos to Andrew: reviewing this show is a logistical and grammatical nightmare.

Though Jackson goes a bit far here and there in his attempts to shock, neat direction from Phillip Breen and excellent performances keep the pace quick and the tone natural, even as the jokes come thick and fast. A surprisingly realistic spin on a strange, familiar trope.

Threeway can be seen at 12.30 at the Pleasance Courtyard, every day until 26 August. For more information and tickets visit the Edinburgh Fringe website.