Lungs, Roundabout at Regency Square, Brighton

A couple stand on the stage, squaring up to each other. They are in the middle of an argument. The Man has just, out of the blue, suggested they have a baby. The Woman, understandably, needs time to adjust to the idea. Particularly as they are in IKEA. In the checkout queue. So starts Duncan Macmillan's very funny and touching two-hander about the disintegration of a relationship.

In 70 tightly packed minutes we see their coupledom move from that engaging first scene, through arguments, lovemaking, failed pregnancy, the possibility of adoption and much more. The couple's initial debate about becoming parents is played out against their concern about the impact of stamping another huge carbon footprint on the world – equivalent to flying between London and New York return every day for seven years, according to the author – but as the play progresses that theme diminishes and the intricacies of the couple's intermingled emotions are increasingly laid bare.

They recycle, they give to charity, they even do fun runs

It's a clever device, as by the end that effort of conscience feels tacked on – rather like the decent values many of us claim to possess but in reality are assumed to make ourselves feel better, or to cause others to think we're nicer than we actually are.

And so it is with Sian Reese-Williams's Woman and Abdul Salis's Man, who talk about grand ideas and ambitions, and what “good people” they are (they recycle, they give to charity, they even do fun runs) but who are prey to the same petty jealousies and lustful yearnings as the next person, and mouth lazy opinions – some people should not be allowed to have children, Man muses – and struggle to act selflessly for the sake of their child.

Macmillan has a lovely turn of phrase – “It's like you punched me in the face and asked me a maths question,” Woman says to Man at his baby suggestion – and puts some juicy meat on to the bones of this entirely believable relationship. And just when we think we know this couple, or their relationship is going along stereotypical lines – Woman wants the conception of their child to be a sacred moment, but Man can only think of the mechanics of sex – Macmillan takes a swift turn and surprises us.

George Perrin directs with verve and the two actors give terrific performances in Paines Plough's 164-seat pop-up, in-the-round touring space, the perfect intimate setting for the piece. It amplifies Macmillan's assured script and we guiltily but thrillingly feel, like the imaginary other customers at IKEA, that we are eavesdroppers.

• Lungs is at Brighton Festival until 22 May, then touring until November