It was the cross-over to end all cross-overs! Finally the various conflicting origins and stories of the DC universe were explained as a Multiverse. The multiverse contained many parallel universes and alternate versions of the characters, with the primary DC continuity referred to as Earth-1. Crisis on Infinite Earths was set to bring these worlds together (and answer a company mandate to make DC Comics more accessible to new readers!).
A cosmic being known as the Monitor supervises many of these realities, but is opposed by his counterpart the Anti-Monitor, who comes from an antimatter universe and begins destroying many of the realities with a wave of antimatter, to replace them with his own.
The Monitor is murdered by his protege, Harbinger, who is possessed by one of the Anti-Monitor’s demons; his death releases enough energy to project the last five parallel Earths into a protective limbo.
The Anti-Monitor recruits Psycho-Pirate to his cause, infusing him with part of his power to manipulate the heroes of Earth-4, Earth-S and Earth-X against the rest; this fails when all five Earths enter the limbo universe.
Harbinger recruits heroes from the remaining Earths to lead an assault on the Anti-Monitor in the antimatter universe, using Alexander Luthor, Jr.’s powers to open a portal between the limbo and antimatter universes.
Pariah tracks down the Anti-Monitor at his fortress, and the heroes destroy a converter powered by stellar energy to destroy the last five Earths; the injured Anti-Monitor retreats and Supergirl dies.
During a lull in the war the villains unite under Brainiac, who kills Earth-Two’s Alexei Luthor while recruiting the Earth-One Lex Luthor to conquer the remaining Earths.
The Anti-Monitor tries to use an antimatter cannon to penetrate the limbo universe and destroy the five partially merged Earths. The Flash (Barry Allen) dies stopping this attempt. A furious Anti-Monitor vows to travel back through time to prevent the creation of the multiverse.
The Spectre unites the heroes and villains by warning them about the Anti-Monitor’s plan; the heroes travel back in time to stop the Anti-Monitor, while the villains travel back in time to the ancient planet Oa to prevent renegade scientist Krona from creating the technology necessary for the Anti-Monitor’s plan to succeed. The villains fail, and Krona continues his experiment.
The Anti-Monitor waits for Alex Luthor to reopen the portal between the positive and antimatter universes, capturing the heroes, but a magically empowered Spectre creates an energy overload which shatters space and time. The five Earths merge into a single shared universe, and the superheroes return to the present; only those present at the dawn of time remember the original realities.
A cosmically empowered Anti-Monitor attacks again, transporting the new Earth to the antimatter universe and summoning a horde of shadow demons. He falls in a carefully planned counterattack culminating in a battle with Kal-L (the Earth-Two Superman), Alexander Luthor of Earth-Three and Superboy of Earth-Prime, with help from New Gods adversary Darkseid.
In this final battle the Anti-Monitor, reduced to a flaming head, crashes into a star and is killed by the Earth-Two Superman. As they are the only four who remember the original past, Alex sends Earth-Two Superman, Earth-Two Lois Lane, Earth-Prime Superboy and himself to a pocket “paradise” dimension.