The main character, Sgt. Mark Stitzer, is a highly-trained member of an elite, if tiny, group of special-forces-type soldiers. They are the V-Force, or Vengeance Force. They exist in case the USSR successfully takes over the US. If this happens, their purpose is to blend into society and wreak our vengeance on the new leadership by using terrorist and gorilla techniques.
Each member of the V-Force monitors popular media (radio, television, print) for a code-phrase assigned only to them. If this code-phrase is repeated enough times over a short period, the soldier is to assume that we've been taken over and their mission has begun.
The only problem is that Sgt. Stitzer's activation code-phrase is unintentionally sent out over public media and so he starts doing his job on the unsuspecting, not-taken-over-by-the-soviets, populace.