The Avengers and the Justice League of America continue collecting the items they have been told can save their realities. They battle each other in the pursuit, while Batman and Captain America attempt to find out who's behind it all.

Across realities, a powerful and mysterious danger looms.

The Mighty Avengers and the Justice League of America, the greatest heroes of their respective worlds, have been dealing with bizarre intruders - villains and monsters from each universe are appearing in the other, on earth and through space.

The Justice League is contacted by the enigmatic grandmaster, a being from the other universe. He tells them both universes are in danger and unless the JLA goes in search of twelve items of power scattered across the twin realms, a universe will die.

They travel to the other earth, finding it a bizarre, unsettling place where monsters and killers roam free, tyrants rule their nations with an iron hand, and innocents die for being genetic mutants. They have little respect for any heroes who protect this world. Superman, in particular, is appalled at the state of it.

However, when they reach the first item - the staggeringly powerful Ultimate Nullifier - they are confronted by the Avengers and hurled back to their own world. They manage to bring the Nullifier with them.

The Avengers encounter the cosmic scholar Metron, from the JLA's world. He tells them of the items of power and cautions them to secure them first, to safeguard their universe.

The Avengers, using technology from Metron, travel to the JLA's world. It is a surprising, unusual place to their eyes, where futuristic cities exist and super heroes are lionized and praised. They don't know what to make of it. Captain America thinks it must be a totalitarian state where superhumans rule the populace, rather than protecting them.

The JLA confronts the Avengers, and a melee breaks out.

What neither group knows, however, is that they are playing a game. A game devised by the Grandmaster to deal with a powerful intruder named "Krona" - who has already destroyed at least two universes, and who is determined to destroy a third.

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