Aliens invade Earth: a well-rounded yarn with a dash of originality--an often-too-conscious salute to Heinlein (Starship Troopers in particular)--from the author of The Man Who Folded Himself and When Harlie Was One. Poor old Earth is in trouble, what with limited nuclear war, disastrous peace treaties forced on the US, and a series of virulent plagues. Then scientists realize that the plagues are part of a wholesale ecological invasion: alien algae are poisoning the seas; tough alien bugs are out-competing their Earth counterparts. And most dangerous of all are the bloodthirsty, nest-building, giant caterpillar-like Chtorrans--though they have no technology and don't seem to be intelligent. Gerrold's plot, then, revolves around naive young biologist Jim McCarthy, who's mistakenly inducted into the Special Forces: to kill a charging Chtorran, he's forced to napalm a buddy; he's sent to Denver to do research, but the relocated government is thumb-twiddling and fending off angry foreigners anxious for technological aid; finally he discovers the secret of the Special Forces (they're actually an anti-Chtorran army-within-an-army), becomes a hero--and, now fully dedicated, rejoins his old unit. Lots of consciousness-raising--often stimulating, occasionally overbearing or mawkish--and sprightly action sequences: a slick, peppy, sometimes engrossing narrative.