There’s an undeniable aura of glamour to any discussion of the Mitford sisters; each as effervescent - and often acid - as a glass of champagne, their upper class eccentricities and wildly divergent life paths make them a rich vein for fans of juicy historic drama. It’s not unfair to say that in many respects they were the English prewar equivalent of the Kardashians, up to and including marriages to self-proclaimed fascists. Amy Rosenthal’s The Party Girls at the Oxford Playhouse beautifully plays on this reputation for privilege and frivolity, as the double meaning of its title implies; there’s a lot of bad politics you can hide under the guise of just larking about.

Glam though the Mitfords certainly are, The Party Girls is about exposing the rot beneath the silks. Told in non-chronological order and in large part through the eyes of communist bête noire Jessica ‘Decca’ Mitford (Emma Noakes), we watch as fascism creeps like a festering wound through the family line, squared against Decca’s later attempts to fight that corruption both in herself and the world at large. Noakes is the play’s emotional heart and she is beating a mile a minute, the civil war between her ideals and her upbringing playing out with perfect clarity in her will-they-won’tthey romance with Joe Cohen’s Bob Treuhaft. And Simon Kenny’s shifting platform set design lends the impression that Decca’s past at Swinthorpe House is constantly in pursuit, or sometimes lurking right behind her.

Of course, if you’ve come looking for arched eyebrows and barbed words, you certainly won’t be disappointed - all the sisters get their share of digs in, but the best come from (who else?), novelist Nancy, played with glorious grand-dame aplomb by Kirsty Besterman. Even convalescing in her Paris residence in 1969 she has the good decency to do it in a silk robe while reading poor Decca to filth, comparing her to a dowdy housewife in a cornflake commercial. Rosethal’s comedic writing is wickedly pointed and a much needed tension breaker amidst the play’s more grim subject matter.

Because in the moments where the Mitford’s Nazism come to the fore, Rosenthal uses the same idiosyncrasies we’ve found charming in them before to illustrate how easily fascism can be laundered into everyday life under the veil of politeness, decorum and ‘good British values’ - the gentility of evil, if you will. People are keen to forget how sympathetic the British aristocracy was to the aims of the Nazi party prewar. Diana Mitford’s rhapsodising about what a ‘hon’ her ‘Kit’ is gets a lot less cute when you know she’s talking about Oswald Mosley, and her final interaction with Decca, a chillingly curt denial of the Holocaust (taken almost verbatim from Diana’s 1989 Desert Island Discs interview), is delivered in a Dior suit lounging on a Parisian chaise, followed shortly by “you did look smashing, darling”. Even the grim portents of wartime in the papers are washed down with socialites’ endorsements of the new vitamin A Pond’s cold cream, a grim 1930s precursor to Instagram feeds alternating between influencers and massacres.

Similarly, Ell Potter’s Unity’s schoolgirlish obsession with Hitler takes on an almost Annie Wilkes-ish turn at points. In a stellar cast, this was one of my favourite performances of the night, as moon eyed crush gives way to dogged conviction and a mania that crosses the line into physical violence. It might seem like a sitcom style set contrivance, but Unity and Decca really did draw a chalk line down the centre of their room so one wall could be plastered in swastikas and the other with Bolshevik posters, an insulated, teenage trivialisation of one of the greatest moral battles in living memory. And truly what better example of the gentility of evil than to watch Unity Mitford agonise over whether Eva Braun is prettier than her?

At a point when we are backsliding to the far right like someone cut the brakes on the world, while at the same time constantly being told to be polite and civil to those perpetrating hatred, it was genuinely refreshing to see Decca’s arc resolve in a way that does not lead to her trying to find some sort of happy medium with her Nazi sympathiser siblings. In her final encounter with Diana the clipped RP accent drops and she yells with her whole chest - where the others forever implore “do forgive”, she orders “do admit”. Rosenthal knows that to offer a fascist a middle ground is to give them the ground entire, and no happy ending lies there. Where it does lie is in the arms of a nice Jewish human rights lawyer who sees all you’ve been as well as all you are, and loves both wholeheartedly.