It concerns what happens when an earthquake cracks open a secret bioweapon buried underground for disposal, and which causes people and animals who breathe it to go a little Ax-Crazy...and Knife Crazy, and Gun Crazy, and Rape Crazy. The main plot surrounds Jon Holman, an Environmental Officer for the British government, who is present at the fog's dramatic entrance and spends most of the book trying to stop the fog; meanwhile, Herbert occasionally takes us on little side trips to see what horrible thing the fog is making happen next.
There is no relation between this novel and John Carpenter's 1980 horror film of the same name.
Provides examples of:
- Almost Lethal Weapons / Plot Armor: The fog drives everyone exposed to it permanently insane, except for the protagonist who was the first one to be exposed to it, and this for some reason conferred lifelong immunity to it after suffering its effects for about a week. He spends the rest of the novel being only one of two people (the other being Casey when she's also cured) who can get close to the fog without succumbing.
- This is given a brief explanation; basically, while he was being treated for unrelated physical injuries he suffered before being infected, the doctors at a local hospital hit on an effective cure via a blood transfusion more or less by accident. Unfortunately, by the time anyone realises this the Fog has spread so far that people are being infected faster than they can be cured.
- Asshole Victim: A lot of the characters who get killed in the vignettes are jerks, or worse (as in the case with the deviants at the boarding school).
- This is also the fate of the scientist who created the fog, having no moral compass. Unfortunately, when he succumbed to the fog's effects in the laboratory, he destroyed all of his notes.
- Ax-Crazy: The natural result of exposure.
- Bury Your Gays: An absolutely heart-wrenching side-story where a lesbian is abandoned by her lover (who is implied to have groomed her somewhat) after the latter is 'cleansed' (i.e. has sex with a man) and goes to commit suicide in the sea, only to have second thoughts but is then drowned by the horde of infected citizens of Bournemouth committing mass suicide, making her death a meaningless statistic and her true reasons forever unknown.
- Crippling Castration: At a boarding school, the Fog drives a PE class to restrain deputy head Summers, who takes delirious sexual pleasure therein; and school gardener Hodges to make discreetly described use of his shears.
- Dangerous Device Disposal Debacle: A lampshade is hung on the fact that all this could have been avoided if the Fog had been destroyed instead of simply buried deep underground, though it turns out that was something of a desperation tactic because nobody could figure out how to destroy it. The unfortunate breakdown in communications that led to a defence contractor dropping their new prototype bunker-buster on the damn thing is less excusable.
- Disproportionate Retribution: Could apply to most of the fog's victims; with their inhibitions removed, they take lethal revenge against their tormentors. Taken up to eleven by a pilot who has just lost his wife to a rival, and flies his airliner into the man's place of work - London's BT Tower
. - Earthquakes Cause Fissures: Played for Horror - a quiet village high street, followed by a baffling tremor, is split apart by a fissure of unclear depth, into which fall two young children. From the depths escapes a strangely driven yellow cloud...
- Elaborate Underground Base: When the fog overwhelms London, high-ranking government officials take refuge in their secret headquarters.
- Face–Heel Turn: Happens repeatedly when characters get a whiff of the gas; in a few cases, it then results in a Heel–Face Turn after the effects wear off.
- Green Aesop: Slight. The fog consumes carbon dioxide to grow, gladly provided by pollution, which is why it tends to drift towards towns and villages, and London.
- The Immune: On John Holman's early exposure to the mycoplasma before its atmospheric dilution, a blood transfusion helped his immune system overcome its effects; enabling him to venture into the Fog for a sample...
- Improbable Infant Survival: Cruelly averted in the very first chapter. One of the fog's first victims is a five-year-old girl, and she dies very quickly because her immature brain cannot handle the mycoplasma ravaging it.
- Kill It with Fire: The mycoplasma's nucleus is destroyed by blowing up a pair of giant gasometers, along with most of the surrounding area.
- Literal Ass-Kicking: A bank manager (who was probably one bad day from Going Postal anyway) affected by the fog starts relentlessly kicking random passers-by in the behind.
- Loss of Inhibitions: En masse and in the worst possible way.
- Police Are Useless: Downplayed and soon subverted. While Detective Inspector Barrow is aggressively suspicious of Holman, Chief Superintendent Wreford more patiently considers Holman's outlandish report, and against the anticipated mass-insanity, takes precautionary measures. When the Fog starts to infect people in their thousands, the Commissioner for Police refers Holman, with his firsthand knowledge of the Fog, to Whitehall.
- Oh, Crap!: Casey and Holman get this when they open their curtains to reveal the fog has engulfed London.
- Rabid Cop: Detective Inspector Barrow gives Holman a hard time. In the final chapter, Holman fights an infected Barrow to the death.
- Unusually Uninteresting Sight: Induced. Holman notes that, despite going insane, some of the fog's victims simply go about their everyday lives like nothing is out of the ordinary, such as waiting at a bus stop amongst the pandemonium.
- The Virus: Actually, a mycoplasma.
- Weaksauce Weakness: The fog is harmed by the hygroscopic compound calcium chloride. This doesn't destroy it though, only redirect it.
- World-Healing Wave: Subverted. When the fog's core is incinerated, it disperses and reveals clear blue sky, but everyone affected is still insane.
- Wrong Genre Savvy: At one point, a looter thinks a World War II gas mask is enough to protect him from the fog - alas it isn't, because the mycoplasma can be absorbed through the skin.
