Judgement Day was a global catastrophe in 2114, when a powerful necromagus called Sabbat raised all the corpses in the world as zombies. Three billion people - including five megacities - died.
It was the first story to feature Johnny Alpha since he was killed off at the end of the Strontium Dog series (from Alpha's point of view it was set two years before his death).
In the year 2178, the sociopath Sabbat has timejumped to escape justice. Strontium Dog Johnny Alpha, who has been in that era before, is sent to stop him before he can wipe out the past. In 2114, Sabbat arrives in Hondo en route to the Radlands of Ji, where he hijacked the leylines and began raising the planet's many corpses.
Judge Dredd is one of the first people to encounter the zombies while leading a group of cadets (including Cadet Giant), on a training mission in the Cursed Earth. Although Dredd leads the cadets back to the relative safety of Mega-City One, Judge Perrier is killed only yards from home.
On his arrival Dredd is informed that the dead are rising all over the world. Judges across the planet begin defending their cities - Judge Armour in Brit-Cit, Judge-Sergeant Joyce in Murphyville, Judge Bruce in Oz, etc. (Brit-Cit only has the northern wastes that aren't bordered by water, but 1.5 million zombies rose inside the city from various graveyards.).
Mega-City One, due to Necropolis, finds itself facing the largest number of zombies: sixty million. Soon into the battle for Mega-City One, Dredd's protégé Judge Dekker is also killed and Dredd has to give the order to set fire to part of the cities' wall to hold back the zombies.
Meanwhile in Hondo, Alpha arrives and runs into Judge Sadu (effectively Hondo's equivalent to Dredd), who assumes he is a criminal. The two soon bond as they hold off the zombie hordes. Hondo uses its sonic cannons, the Skreemers, to hold the zombies back - a power drain that won't last long.
The Hondo authorities organise and host an international conference of chief judges from all over the world to decide how to deal with the zombie threat. It is attended by Dredd and Chief Judge McGruder, who leaves Judge Hershey in charge of the city in her absence. Alpha and Sadu also attend, Sadu by now being convinced that Alpha is on the level.
The conference is interrupted by the unexpected intrusion of Sabbat himself, who teleports in to informs the Judges of his plan - to kill everyone in the world and then use the army of zombies to conquer the galaxy - and offers them a "quick death", taunting them that entire cities have already fallen. Dredd shoots Sabbat in the head, apparently causing a mortal wound, but to no effect, as Sabbat simply removes the bullet and vanishes.
Reports come in that Brasilia, South-Am City, Sino-City One, Djakarta, and Mega-City Two are indeed overrun. Dredd proposes that instead of allowing the cities' populations to serve as more undead soldiers for Sabbat, they should be annihilated with nuclear weapons, even though there may still be survivors trapped within them. Although horrified by the plan, the chief judges agree to carry it out, with the loss of two billion lives.
Once scientists figure out where Sabbat is based, Dredd leads a suicide mission to assassinate him; a striketeam of attending foreign Judges are drafted, backed up by Hondo's Samurai battle armour. He bans Alpha from attending because he is a mutant and a wanted criminal in Mega-City One. Alpha proceeds to knock out Judge Joyce and take Joyce's place aboard Dredd's spaceship, his face concealed by a Samurai visor.
Dredd's team are forced to parachute from low orbit into Sabbat's lair, as Sabbat is using his powers to prevent all flying vehicles from working outside the mega-cities. They land in the radioactive Radlands of Ji (in post-nuclear China), where most of them are slaughtered. Judge Bruce dies in Alpha's arms; out of the twenty who started the mission, soon only Dredd, Alpha and Sadu are left, and are taken prisoner. Meanwhile, the death toll reaches three billion and Casablanca and Bangkok report they're about to collapse. The delegates in Hondo join the fight for that city, while every single Mega-City One asset is desperately trying to keep the zombies out.
While Sabbat taunts his prisoners about his imminent victory, Sadu manages to escape and release Dredd and Alpha, but sacrifices his life in the process. After a lengthy fight, Dredd and Alpha eventually manage to decapitate Sabbat, and leave him helpless, powerless, but still alive (although a story published three months later reveals he has lost his mind and is reduced to a drooling vegetable). At the moment of Sabbat's defeat, his zombies instantly "switch off" and collapse all over the world at the eleventh hour.
In recognition of Alpha's help, Dredd recommends that he be pardoned for his earlier crimes. However they still face a long walk back to civilization through hundreds of miles of radioactive desert populated with hostile mutants and outlaws. Dredd is optimistic about their chances though, saying "Who the hell's gonna mess with us?"
Judgement Day triggered a simultaneous attack by the psychotic mutant Gabriel, who had created an army of monsters out of human victims. His greater plan, taking advantage of the destruction, was to himself wipe out all life on Earth with mutagenic spores except for Brit-Cit, which he was intending to conquer and turn into an eternal Hell. Armitage and a small team stopped him in time. 
In the immediate aftermath, a Sino-Cit nuclear silo almost hit Hondo with a pre-programmed retalitator assault from Sino-Cit One's destruction. Dredd and Alpha prevented this from happening, and Alpha was returned to his home time afterwards.
With every Judge focused on the external threat, crime lords and gangs had seized control over parts of the city, such as Sector 123, and mutants gangs made constant raids due to the gaping holes in the city's walls. The initial aftermath would involve regaining control of the lost sectors and constantly fending off mutant assaults. As a result of Sabbat's actions, there would be occasional spontaneous zombie risings for a while after.
The loss of Judge life - coming so soon after Necropolis - led to problems across the southern sectors, declining morale, and cadet Judges being rushed onto the streets. McGruder would respond to this with the Mechanismo project: robot Judges. Other Mega-Cities would stop sending Judges to Luna-1, which caused the colony's Judge force to suffer as a result.
Hondo began rebuilding Mega-City Two (calling it the Hondo cluster) so as to move many of their citizens over there  but was forced to abandon the project.
The Americans  revealed that Mega-City One's Psi Division had gleaned knowledge of the 2150 nuclear world war from Johnny Alpha's mind, recording this data in the "Alpha File." The information was kept hidden from Brit-Cit.
In 2133, Judge Dredd would go there to prevent a unified mutant army from gaining Justice Department technology.
The format made no concessions to those who only bought one publication, as the story was entirely linear, with two episodes a fortnight in the weekly 2000 AD followed by a third episode in the (then) fortnightly Megazine. The editors attempted to address this problem in the next two crossovers, "Wilderlands" (1994) and "The Doomsday Scenario" (1999), by having two separate plot threads in each story, one in each comic, so that readers who only bought one could still follow the story. Since then, no mega-epic has crossed over into both comics.
Garth Ennis himself has criticised his own story, telling Thrill Power Overload "bits were okay... I recycled far too much material from other epics", particularly the Apocalypse War. He was even more scathing a decade later when talking to The Mega-Collection for their Judgement Day reprint, saying he'd been "floundering around without the wit or experience to come up with anything decent" and given a "feeble villain" with "incredibly repetitive" zombies. "As for the scenes where the cities get nuked, who cares? The sheer drama of Part 23 of The Apocalypse War makes the sequence look like a series of damp farts." 
- The GCC in Alpha's time are afraid that Sabbat will alter the timeline. As far as we can tell, the events of Judgement Day don't do this; certainly Alpha's not concerned about it. (Strontium Dog's continuity with Dredd is quite shaky, and John Wagner - who wrote their frist crossover - has said he doesn't view them as linked)
- Various cities are retroactively absent from the Hondo summit as they hadn't been created or their Judges designed yet. The book Cursed Earth Asylum would explain the absence of Sino-City Two as the Sino Block deliberately being away, "declining all assistance", and were greatly unhappy when the other cities bombed Sino-City One.
- The story features some of the first published artwork from Chris Halls, an early pseudonym of the acclaimed director Chris Cunningham.
- The origin of Sabbat is a vicious spin on Walter the Softy in The Beano.
- ↑ Megazine 2.64
- ↑ Meg 2.6 to 2.71, Armitage: City of the Dead
- ↑ Judge Dredd: Pre-Emptive Revenge audio drama by Big Finish Productions
- ↑ The Taking of Sector 123, Megazine #2.10-11; The Marshal, 2000AD progs 800-03
- ↑ The Kinda Dead Man, 2000AD prog 816)
- ↑ Prog 826: "Unwelcome Guests"
- ↑ Breathing Space, 2000AD prog 1451-9)
- ↑ Wetworks
- ↑ Jihad audio
- ↑ Chopper: Supersurf 13 2000 AD progs 964-971
- ↑ Judge Dredd Megazine #283
- ↑ "California Babylon", progs 1731-4
- ↑ "Thrill Power Overload!", page 157
- ↑ Mega-Collection Volume 37
- ↑ Cursed Earth Asylum