Parasite is a Korean episode of Inside No.9 which like the BBC Pemberton and Shearsmith anthology series, starts with an interesting contrivance then works backwards to intricately facilitate it. This clockwork universe approach to storytelling makes for a satisfying act of on-screen contortion, as those carefully plotted twists are experienced in linear fashion. But the design is inevitably built on a series of improbabilities. A realist narrative would inevitably feel less contrived, be messy even. A movie like Parasite can’t afford such ambiguities or loose ends, because it’s a polemical puzzle box with a single, pre-ordained solution.

Styled as a black comedy, the movie’s real concern is the chasm-sized gap between South Korea’s richest families and her poorest; a point Bong Joon-ho rams home in a series of memorable comic vignettes – the Kim family opening their windows so they can be fumigated along with the street, the daughter sitting on the family toilet to contain the sewage-infused overspill from local flooding, a subterranean neighbourhood that’s particularly vulnerable to said flooding, in contrast to the modern house in the elevated part of town where the entitled Park family reside.

Anyone who’s had the misfortune of being disempowered on their work life, will recognise the naivety, soft condescension and pathological self-absorption that characterises the Park family, particularly the aloof patriarch. Bong’s careful to paint them as cosseted and self-important, rather than monstrous. In the first half of the movie this even allows for some sympathy as the Kim family scheme their way into service and caring positions within the home; support available to the rich that the less fortunate simply have to do without.

Later, with the infiltration complete, we’re left to wonder where the story has left to go. The Kims are simply trying to tap into the Park’s wealth and comfort; they’re neither callous or cruel enough to consider bumping off their employers or trying to appropriate their identity.

Bong’s second-half solution is to double down on the idea of invisible people and their dependents; the idea that the super-rich wouldn’t recognise the plight of the needy if they were living right under their nose.

A house with a literal sub-basement housing the dispossessed, whose life line has been inadvertently cut by the Kims, is a darkly funny idea. It’s also a highly improbable plot contrivance.

Do we really believe the Parks could have bought a uniquely designed house built by a famous architect and not been told that the place contained a labyrinthine bunker for survival in the event of a North Korean nuclear strike? Or that the old housekeeper could have successfully fed and maintained her husband, who resided therein, without anyone bar the Parks’ young son noticing? This is the scenario you have to buy into for Parasite to work, but if you can bury your doubts it’s a pointed and entertaining social satire with assured execution.

Boon’s film scores highly when it comes to the incremental building of tension, and it won’t be lost on the audience that it’s the Kims audacious meddling in the South Korean social order that unleashes anticipated reprisal and traps them in a purgatory of their own creation; a scenario where only by elevating themselves to the status of their former employees can they liberate themselves.

There’s no beating the system, Boon seems to say with a weary sigh, only its temporary disruption. South Korea’s colonial complex manifests itself in the namedropping of US business and education, as well as the spectre of Western European immigration; factors, Boon seems to be suggesting, that help entrench the American-style poverty gap the film disdains. One’s left to wonder who’s parasitically living off whom? The Kims off the Parks, or the Americans off their anti-communist allies?