I don’t know about you, but I get a real buzz out of walking into a theatre I’ve already visited and having difficulty recognising the space. It’s something about being duped, or thrown into a new environment that you weren’t expecting, that pointedly throws your preconceptions about a show out the window. Serial offenders of this include the Young Vic and the Gate, but the other night I walked unsuspectingly into the tiny Finborough Theatre to find the space transformed into a living room.

In an unusual form of rep, the set is used for both Laura Jacqmin’s A Third and Alpha Beta. Which is great, because a set that really adds such uncertainty and intrigue for the audience deserves to be seen more than once. We are told we can sit anywhere but the grey sofas, and actors sit uncomfortably close and move us out their way at their whim. It’s a shame the publicity for A Third doesn’t capitalise on the unique little experience this set creates; it transforms a seemingly straightforward play into something precise and thoughtful.

A Third is a play about threesomes. The format of a couple wanting to spice up their marriage is by no means a new one, having been seen before in plays like Mark Canton’s Jezebel. But Chicago-based playwright Jacqmin crafts a funny and frank look into sexuality that actually seems to be making a point: that by inviting the outside world into their marriage, the couple inevitably rip their relationship in two.

The characters, whether as a result of Jacqmin’s text or the actor’s characterisation, often seem in-between two places. The couple in question, Allison (Asha Reid) and Paul (Jeremy Legat), seem in their actions to be liberal and adventurous, and yet don’t like making a scene and have difficulty talking frankly about sex in public. It leaves a great deal of uncertainty as to where we stand with the lead characters. Nonetheless, Legat gives a great performance as Paul, whose motivations are clear as day, and there is good support from Will Alexander and Lucy Roslyn as the eponymous thirds caught up in Allison and Paul’s exploits.

As the living room set becomes a club or a café, a restaurant or a bus stop, it soon becomes evident why director Josh Roche has chosen to stage the play in this way. Not only does the outside world trespass on the couple’s relationship, but so do we, the audience. Slightly distanced or fragmented though this look at sexuality may be, it’s fitting for the audience who are only there for a short time and intrude on the marriage just like the thirds do. It means that the slight spark of hope at the end of the play comforts us slightly – perhaps when the world leaves their living room, or the audience leaves the theatre, the couple will be able to patch things up in private.

A Third is playing at Finborough Theatre until 20 July. For more information and tickets, see the Finborough Theatre website.