At the end of Annihilation, Galactus destroys the invading Annihilation Wave with enough force to destroy star systems and other cosmic beings. The blast was felt far and wide even across other universes and in the far world of Apokolips, the mad dictator Darkseid feels it. Power like nothing he ever tasted. Power he must have.
The Guardians of the Universe and Highfather of the New Gods also feel it and are concerned. They send their best men to investigate, Hal Jordan, John Stewart, and Guy Gardner of the Green Lantern Corps and Lightray. After coming to the Marvel Universe, Nova immediately finds the Green Lanterns and engages them in a fight until the quartet are attacked by a group of Darkseid's parademons. The four have great difficulty until Lightray shows up and turns the tide. After the battle ends and things are sorted out, Nova leads the DC heroes on a path to Galactus while Darkseid watches due to a tracking device the parademons implanted on Nova.
While the DC heroes and Nova meet, Adam Strange is on a mission to discover who's been using the Zeta Beam besides Rann. With help from his fellow Forgotten Heroes, Starfire and Animal Man, he's had no luck searching the cosmos. Discovering a new area of space supposely inhabited, the group travels to the Marvel Universe and meet Quasar and Moondragon. Luckily, due to Starfire, they do not fight and Quasar and Moondragon agree to help before the group is blindsided by the Magus.
When Nova, the Green Lanterns, and Lightray meet Galactus and the Silver Surfer, Darkseid and his minions attack. This battle, along with the Magus battle, creates problems through the DC and Marvel universes until a schism is created seperating the battles. With the schism, John Stewart, Animal Man, and Starfire end up stuck in the Marvel universe and with Quasar and Moondragon in the DC universe and the rest of the characters in their respective universes.
With the battles still going on, other space super-heroes join in the fight with them switching universes as well. Galactus is able to stretch himself to fight in both battles and ends up sacrificing himself to stop Magus and Darkseid. As a Zeta Beam sends the DC heroes back to Rann, another Zeta Beam gathers the energies lost by Magus, Darkseid, and Galactus and sends, in the form of the Ultimate Nullifer, to the Watcher as he aides the Fanstic Four against Galactus for the first time.