The year is 2494. For seventy years, the Earth has been under siege by the insectoid Sept. Since the siege began, the Sept have been kept at bay by a huge Force Sphere which surrounds the Earth and the Moon. However, the Sept are on the verge of discovering how to breach the Sphere unless it can be strengthened according to the formulae contained in the original plans. Unfortunately, these plans are concealed somewhere in the Martian city of Marsport, now Sept-occupied and booby-trapped by the city's M-Central computer. One man, Commander John Marsh, has been sent to retrieve the plans in the belief that a determined man can sometimes achieve more than an army.
