Captain Oates' Left Sock, Finborough Theatre, London

A play whose audience barely outnumbers the cast is a byword for failure, but at Captain Oates' Left Sock this happens by design. The seating has been removed from the Finborough Theatre to create a bare room with a ring of chairs, backs to the wall.

Fifteen are for the actors who portray a doctor, two nurses and the members of a mental-hospital therapy group, and 20 are for the customers. Although Russell Bolam's staging might seem to ensure a full house each night, John Antrobus's play of 1969 makes it likely those actors will be playing to empty seats.

Antrobus was one of the writers of The Goon Show, and his many plays include The Bed-Sitting Room (a collaboration with Spike Milligan), in which a character is transformed into one. If the mood here is not surrealistic fancy, a conventional comedy, at least, seems to be brewing – the cast includes an elderly colonel, a young rebel, a sexy girl and a hearty man who laughs off the charge that he is interested in the girl with "I'm old enough to be your father". There is a farce-style mystery (someone has been stealing the women's underwear), and there are some funny lines that parody English understatement. When the angry young chap jumps about, waving his penis, a middle-aged woman has no reaction until he says "bloody" – then she is shocked and censorious: "It set me back. I was ready to be discharged." "Well," the mostly silent doctor remarks, "young David has certainly put the cat among the pigeons."

For the most part, however, Captain Oates is not so much comical or satirical as drearily realistic, with patients who are, as in life, whiny, repetitious, sullen, and banal. Several say only a few words, one says nothing, and we never find out the troubles of the colonel or the woman who is set back. The extravagant size of the cast is unmatched by any verbal or emotional extravagance, and the two moments of intensity – David's willy-waving and a later threat of violence – feel awkward and forced.

Sitting next to actors portraying mental patients should be the perfect situation for demonstrating – with tenderness, savage humour, or fear – that the mad are among us and not unlike us. Unfortunately, Captain Oates shows us only what we all know from hearing people talk to themselves in public, whether or not they are using mobile phones – that the mad can be as boring as anyone else.

To 31 January (0844 847 1652)