After a year of combating a pandemic with relatively benign, cold-like, symptoms, the frustrated populace of Taiwan - many considering the virus a hoax perpetuated by the government or big business - finally lets its guard down.
This is when the Alvin virus spontaneously mutates into a rabies-like plague, unleashing its victims' inner demons.
As random acts of violence begin to break out across the city, the streets of Taipei escalate into an orgy of brutality and depravity, as those infected are driven by their most primitive desires to commit the most extreme acts of cruelty and depravity they can imagine.
A young couple, Jim (Berant Zhu) and Kat (Regina Lei), find themselves separated in the chaos.
Kat was taking the underground to work when the rioting began, while Jim was getting his morning coffee at a local café.
Trapped between the trigger-happy authorities and tidal waves of ordinary people transformed into psychotic killers, as personified by the lecherous businessman (Tzu-Chiang Wang) who stalks Kat, The Sadness follows Jim and Kat on their separate odysseys, as they attempt to survive and reunite.
A strikingly memorable 99-minute movie, The Sadness is the first feature film of Canadian writer/director Rob Jabbaz, who works out of Taiwan, and, if nothing else, signals the arrival of a major new horror talent on the scene.
It needs to be stated up front that this is not a film for the overly sensitive, easily triggered, or faint of heart, as it very quickly descends into epic Grand Guignol, with spraying blood and viscera decorating many a scene.
The early subway sequence is simultaneously over-the-top and terrifyingly real in its depiction of human ferocity in a confined space that you may never want to travel on a tube train again.
The Sadness is George Romero's 1973 film The Crazies cranked up way past 11, with its pseudo-zombie infected retaining their intelligence and ability to communicate but with no filters, no restraint holding them back.
Like something from a Clive Barker splatterpunk opera, the plot of The Sadness is about those grim, dark thoughts we all experience upon occasion - but would never act upon - being made manifest.
The mutated Alvin virus eliminates those social contracts of human civilization, allowing the most evil and sadistic part of a person's id to take the driving seat.
With Jabbaz's commitment to the verisimilitude of the plague he has created at the heart of The Sadness, sexual violence is threatened (often) and suggested throughout the film, but never graphically shown.
The harrowing nature of the horror is heightened from more mainstream zombie outings because the fast-moving infected can still talk, issuing vulgar taunts and gross threats of their intentions.
These plant disturbing images in the viewer's mind which never manifest on camera, but help accentuate what we do see: the boundary-challenging display of dismembered, often cannibalised, bodies, severed limbs, torn flesh etc
The titular "sadness" is both from the realisation that the infected know what they are doing, but can't stop themselves (which is why some of them are seen crying before or after their attacks), and also the psychological effect this scenario has on those not yet affected by the virus.
As the gorefest snowballs, the uninfected are pushed to shocking acts of violence in self-defence, so that eventually it's difficult to differentiate between the infected and uninfected.
Some of the manic momentum of The Sadness is lost towards the end of the second act, during a key scene that (necessarily) exposits the backstory of the virus and how it works, before the story turns inwards for its conclusion, bringing the drama down to a very intimate level.
It gradually becomes clear, as the movie enters its third act, that we are not heading towards a happy ending, rather something far more nihilistic that borrows a trick from a particularly famous zombie movie (just styling it differently).
As infection/zombie tales go, The Sadness is not so much a film of be enjoyed, as it is an experience.
What you make of that experience depends on your intestinal fortitude and willingness to watch a brilliantly shot survival horror film set against a bleak, misanthropic, backdrop of disgusting cruelty being perpetrated by the worst of humanity.
However, The Sadness isn't simply a bloodbath of mindless violence (although there is rather a lot of that), the script also cleverly addresses - very topically - the seemingly modern phenomena of social media-fuelled paranoia and politicised truth that surrounds wide-spread emergencies, such as pandemics.
Be prepared to be challenged.