One might be forgiven for thinking that a two-hander about alcoholism is going to be a tough watch. Yet David Ireland’s short, sharp play is hilariously funny, until it isn’t. It is also generous, warm-hearted and abrasively realistic.

The combination of Jack (Slow Horses) Lowden and the supernaturally versatile Martin Freeman is an onstage marriage made in heaven. Lowden is Luka, a Scottish incel whose failure to have a relationship with a woman has driven him to a life of excessive drinking and self-abuse. Freeman is James, a recovering alcoholic who is married with a child and who becomes his AA sponsor.

As details of Luka’s addictive and pathetic lifestyle emerge, James becomes increasingly concerned about the welfare of his client, as well as facing severe doubts about his own domestic situation.

At first, James seems wise and eager to help, gently parrying Luka’s aggression and frustration while probing his background. Luka responds fitfully until he has a religious epiphany at the gym in which he fantasises competing with actor Willem Dafoe on a treadmill.

As a result, Luka starts to get evangelical on his sponsor, who despises all organised religions and promotes his ‘spirituality’ through a half-hearted Buddhism. With just a handful of plastic chairs and a table with coffee and tea things, Finn den Hertog’s in-the-round production relies almost entirely on the performances, although credit is due to Lizzie Powell’s lighting and Mark Melville’s music and sound design.

The banter is waspish and very funny, enhanced by the actors’ body language and double takes that deliver as many laughs as the dialogue.

Ireland has consummate control of his narrative and his slippery wit. When Luka lies about drinking coffee in a pub and James ripostes, “You don’t go to a brothel for a kiss,” is just one of many zingers. In addition, James’ suggestion that Luka should take up weights or knitting in place of masturbation is almost as funny as his later appearance in a pair of bunny ears. It is a Harvey moment that I won't be forgetting in a hurry.

Lightweight, fast on its feet, it is far from frivolous and might have benefited from being longer. That’s not something I say very often.

THE FIFTH STEP IS AT @SOHOPLACE UNTIL JULY 26