There is a famous episode of Porridge, the seminal, prison-set sit-com from the seventies starring Ronnie Barker, in which the two main characters, Fletcher and Godber, spend the entire episode in Fletcher’s cell, in real time, just talking to each other. There’s an equally famous early episode of Seinfeld in which the characters do something similar: they spend the entire episode, in real time, waiting for a table in a Chinese restaurant.

Both of those programmes went down in history because of their bold starkness. They rejected, for one episode, the normal expectations of their genre, and managed to create a petri dish of pure sulphuric comedy.

Window Seat is a bold attempt to do something similar.

Two characters, a mother (Trix) and a daughter (Lois), sit down next to each other on a flight bound for Florence, and the entire play (apart from the odd loo break) takes place with them in those seats, waiting for the plane to take off. Over the course of an hour they delve into family relationships, ambitions for the future, regrets about the past, love, art and hidden tattoos.

It’s a powerful and highly theatrical premise. The economy-class seats face directly at the audience, and most of the dialogue is delivered straight out at us too. At times it feels almost as if they are the audience and we the play. Who watches the watchmen? And there are lovely little touches like the surreal announcements from the captain (flight delayed by geese sunbathing on the runway), and Trix and Lois tripping over each other’s knees as they squeeze in and out of their seats.

Cleopatra Coleman is a first-time writer and director, and mounting a play that is basically all dialogue, with no plot and no action, is an extraordinarily brave decision for one’s opening attempt. While it’s not an out-and-out triumph, it’s still pretty damn good: an impressive attempt at something that veteran writers with years of experience might only try after years of honing their craft in less nakedly exposed vehicles.

The reason those pared-down episodes of Porridge and Seinfeld worked so well was that their scripts were screamingly funny, they featured characters that were already known and loved by audiences, and there was also a subtle plot-line snaking through the episode, building to a satisfying climax. Window Seat doesn’t have all that. It can’t. Nevertheless, the dialogue, while perhaps not sparkling with wit, is certainly gently bubbling. And the characters do grow on you. Trix, the mother, is slightly out-of-touch but still pretty cool, and Lois, her daughter, is a volcano of alternating indignation and affection. Marianne Nossair and Avanthika Balaji are a gloriously talented double-act of love and resentment.

There is one sequence, in which Trix reminisces about her first trip to Florence, when a story genuinely flickers into life, and the play feels a lot easier and more natural as a result. In fact, at that point, Lois symbolically raises the arm of the chair that separates her from her mother, and for a few minutes they share a true bond of intimacy.

But like the aeroplane, these characters’ vehicle just doesn’t quite take flight. It puts self-analysis above entertainment. It fits its own oxygen mask before helping others. That may be OK in a plane, but in a theatre the audience comes first. Coleman clearly has real talent as a writer. Now she just needs to get her characters to do something.