Morgan's home computer spouts ""garbage"": his friend Sid, along with scores of other local residents, sees strange pinwheels in the sky; the power fails; fires break out all over town. . . and a strange-looking alien called Nimbus appears at Morgan's mountainside home. Nimbus says he's a Spartican come to save the world from humans' race toward self-destruction. Their first target is a nearby nuclear plant, and Morgan and Sid are taken up in the space craft that will drop a sort of time bomb on the plant. Morgan and Sid, high school seniors, have opposed the plant; but now, pretending to accept a promised role in running the world for the Sparticans, they secretly sabotage the bombing. Corbett never does explain who the aliens are and how they got here, but if Morgan and Sid are right they are from ""inner space, not outer""--giant ants come up from inside the mountain. Fantastic, but not especially ingenious--and Corbett uncharacteristically burdens his slight story with too much earnest talk.