Thanos has come to the DC universe, as he has built a weapon -- and only the last Green Lantern's ring can activate it. In testing a rift between the two universes, Thanos pulls the Cyborg Superman into the Marvel universe, and sends Terrax the Tamer into the DC one. Meanwhile, Parallax has traveled to the Marvel U to pursue Cyborg, and runs into the Silver Surfer in combat with him.
Thanos "saves" Kyle Rayner, the Green Lantern, from Terrax, and convinces him to aid him in stopping Parallax from destroying both universes. It is not until after he has gained the power of a destroyed Oa that Thanos reveals his true plan -- to destroy all creation, and be with his love, Lady Death.
Parallax convinces the Surfer to give him the Power Cosmic to stop Thanos, but in the end, it takes the combined efforts of Green Lantern and the Silver Surfer to defeat the two mad villains.