MAZINGER U.S.A.

Set at beginning of the 100 Years War in 220X, sometime after World War III devastated 90 percent of the global population, "the surviving nations have realigned into northern and southern superpowers."

What remains of mankind have burrowed deep underground to escape the deadly radiation that permeates the surface. In order to continue fighting, the militaries of the world developed special gigantic combat armor, the strongest of which, Mazinger, is piloted by Major Koji Kabuto.

After a great battle, Mazinger gets caught in an enemy explosion that rips a wormhole in space/time that sends him to a world of giants, called Lithgor.