From the back cover:

DOOMSDAY CAME ABOUT EVERY FIVE YEARS...

Someone Out There really hated humans. Twenty years have passed since Shiva I first swept aside Earth's crude defenses and rained down destruction. Now Shiva V has entered the Solar System, more powerful than any of its predecessors.

The Shiva cannot be destroyed by fleets of ships: we tried, and it was the fleets that were destroyed. It cannot be defeated by a clandestinely developed superweapon based on new principles of physics: no such weapon exists. It cannot be defeated by a forceful American President and his faithful generals: they do not know what to do.

There is only one way to defeat a Shiva: get inside and kill it. Once again, in the personae of five champions, four billion of us are about to do just that.