The Unfriend review – manners can be the death of you in Steven Moffat’s comedy

Inside number nine on a west London street live Peter (Reece Shearsmith) and Debbie (Amanda Abbington), a timorous couple who are “dying of manners”. They can’t communicate with their teenage children Alex (Gabriel Howell) and Rosie (Maddie Holliday), can’t stand their droning neighbour (Michael Simkins) and can’t believe they have agreed to host a houseguest – or, as Alex describes her, an “unscheduled obstacle”.

This is Elsa (Frances Barber), a brash American who was amusing company when they met on a cruise, but whose arrival prompts Peter to yearn for a real-world equivalent of Facebook’s “unfriend” button. Hence the title of this debut play from that TV Midas, Steven Moffat, directed by his Sherlock/Dracula cohort Mark Gatiss, and worthy (high praise, this) of a place in Shearsmith and Steve Pemberton’s macabre comedy series Inside No 9. For Elsa isn’t just a burden. She may also be a murderer, if the internet rumours are to be believed. Still, that’s no reason to kick her out, is it?

Barely maintained boundaries are everywhere in Moffat’s script. The unresolved matter of a crumbling garden wall looms over the family’s interactions with their neighbour. Peter won’t allow Alex to break wind in the living room, banishing him to “less public” areas of the house, while Debbie reprimands Rosie for eavesdropping. Into this knife-edge existence sweeps Elsa, a tornado in a Gucci scarf, who smashes through all the politeness (“You’re a little bit passive-aggressive, aren’t you?”) and encourages the sullen children out of their rooms as their parents look on in disbelief.

Despite her dubious political leanings, Elsa is not exactly a Trump surrogate, even if her burnt-orange velour tracksuit does call his complexion to mind. Moffat is more concerned with the idea of what seemingly moral people will countenance in exchange for short-term gain. The decline of standards occurs incrementally; before you know it, accommodating a killer (if that is indeed what Elsa is) doesn’t seem all that bad. “Give it a year and we’ll be voting for her,” predicts Peter, who reads this newspaper just so he knows what to be angry about.

Gatiss’s brisk directing doesn’t permit lags or reflection (he was asked by prospective producers: “Is it just funny?”) and the rollicking pace plays to the strengths of cast and text alike. Simkins gives a bravely unshowy performance calling on reserves of skill and stamina. Barber breathes new life into a dynamic – devil-may-care American tutoring prim Britons – which feels vaguely played-out.

Related: ‘None of it has felt insurmountable’: Amanda Abbington on Sherlock, separation and her fiancé’s nightmare fall

This is Shearsmith’s evening, though, his paroxysms of awkwardness becoming ever more finely calibrated as the hysteria escalates. Gesturing authoritatively with one arm, he is reduced to an ineffectual usher in his own home. Maintaining two separate lies at once, he seems poised to rip himself in half. The show’s farcical high-point, in which he confronts something far nastier than a fart in the front room, is an agonisingly protracted solo number of exquisite embarrassment. It’s a pity Abbington isn’t given a larger share of the physical comedy, remaining mostly a bystander or straight woman, which seems a waste of her gift for precision.

All this chaos is contained, just about, within Robert Jones’s appropriately higgledy-piggledy set. The bottom half is a cross-section of kitchen and living room, whereas the upper level seems to twist slightly away from us, giving the impression of an otherwise unremarkable hat worn suspiciously askew.