The basic premise of the film is thus:
In the dark ages, 'The Enemy' sent an evil machine to Earth. This machine would convert the dead and dying of the Human race into 'mutants', who were really just fast zombies with one arm turned into a big blade, and use them to destroy the world. A hero rose and drove the Zombutants back, and sealed the machine beneath the Earth forever. This hero formed a secret order called The Brotherhood, who would guard the seal for eternity..blah blah blah.
Fast forward to the year 2707. There doesn't seem to be any countries left in the world, only the Four Big Corporations, who were constantly at war for control of the limited resources of Earth. During a battle, the seal holding The Evil Machine is broken, and it starts making Zombutants again. Apparently the Four Big Corporations, who have some serious military space age weapons at hand cannot stop this wave of zombies whose only weapons are a sharpened forearm and are quite tough, so they decide to evacuate Earth and go live on Mars. Ron Perlman, leader of the brotherhood, decides to go down into the hole where the seal was broken and blow up the Evil Machine with about 6 random soldiers, a mute ninja priestess and a bomb that apparently was designed to fit into a hole in the front of a panel built into the Evil Machine. Because, y'know, all doomsday devices come complete with a bomb that is the only thing that will destroy it which is always left in the hands of the monks sworn to stop The Enemy.
Yeah, it is as bollocks as it sounds.