Zero 3.0. Mech. Chronicles, #3

With the mechs guarding planet Beetar for a century, they failed to notice new enemies coming for them. One is an advanced alien species from another galaxy and the other is a former ally they thought was long dead. The aliens enigmatically vanish, promising to return to vanquish the mechs once they've properly prepared for battle, but the reprieve is momentary when the newest threat emerges. The man who created them has come back to finish them off once and for all with a small army of his own creation.
Can Zero and a handful of rusty old mechs defeat a foe who has been preparing for their demise for a century?