The Crew of a Sentinel unit (a 30 ton walking tank) is sent on a desperate mission to protect the flank of a retreating army from alien invaders.

Late in the 20th century a great plague swept the earth, decimating humanity. In 2017 the Zisk an alien humanoid race, arrive and cure the plague, saving mankind from extinction. But the Zisk are not the benafactors once thought. They are out to shatter the newly formed Unified Commonwealth. After 75 years of bloody civil war it is up to the Commonwealth's army of sentinels to defend the earth from the evil Zisk.—Mike McGee