This is gonna be beyond cheesy but here is my explanation!
Gigan 2004 version is severely injured but makes it back to the Xilliens, they cant rbeuild him as he was but they can revert him to his original Gigan form!
Gigan then signals to all the evil monsters to destroy Godzilla, tokyo and New york!
Godzilla 2014 asleep at the bottom of the sea heres these signals and rushes to the shore to warn Rodan Anguirus and Mothra!
Godzilla 2014 by doing this get bombarded by the monsters we gave like, Hedorah, Gigan the original, and they are led by Mecha King Ghidorah! Godzilla 2014 fights off the 10 foes but is inevitably defeated by Mecha king ghidorahs special beam.
Mothra arrives to late but sees Godzilla being carried off unconcious into the xillien mothership so he can be transported to Planet X.
Mothra flies after them and tries to save Godzilla but is defeated by a point blank Gigan eye beam.
A dieing mothra falls on the ground near tokyo!
Rodan Anguirus arrive at the scene and Mothra tells them, "Save earth!"
Just as mothra hears this Hedorah arrives and their first fight begins!
The story should end as Godzilla is dropped off at Planet X and is confronted by Mecha King Ghidorah.
Godzilla 2014 and mecha king ghidorah have one final battle for the fate of earth!
Godzilla 2014 wins and uses Mehca king ghidorahs space ship heart to teleport back to earth.
