Earth in the 23rd century. Now a federation of cultures and ethnicities, humans have left the boundaries of their planet to head into space for the time-honored purpose of colonization. While out in the great beyond at the farther reaches of the galaxy, colonization has resulted in human explorers meeting up with a new species of being that resemble spiders more than people. On their world known as Klendathu, these creatures that are colloquially termed “Bugs” do not appear to have benefited from intellectual evolution quite as much as they have benefited from evolution of the ability to become the perfect killing machine.
The Federation of Earth high praises their own well-trained killing machines. Joining the military and becoming a killing machine earns those who make this decision the opportunity to enjoy greater privilege than others. Johnny Rico, Carmen, Carl and Dizzy Flores decide to take advantage of this affordability of opportunity and enlist in Federal Service quickly upon graduation from high school.
Carmen is a pilot. Carl joins Military Intelligence. Rico is a grunt soldier in the infantry and Dizzy is right there with him because, well, she is crazy about the boy. As they all undergo their assorted paths of basic training, Rico nearly quits after essentially having the guilt of being involved in accidental friendly fire incident beaten out of him. Or the attempt to have it beaten out of his, at any rate. Later, Buenos Aires—the home of the high school kids turned military units—is destroyed by an asteroid. When it is reported that the asteroid was actually the work of the Bugs far away, Rico stops the paperwork on quitting and everyone joins in the launch of an invasion force to exact vengeance and retribution upon the creatures of Klendathu.
Retributive justice begins as a complete disaster. Hundreds of thousands die and Rico takes a pretty bad hit to his leg. He is accidentally listed as being killed which causes Carmen—who also loves the boy—no end of tragic heartbreak. In actuality, Rico and Dizzy have been reassigned to the “Roughneck” elite commando squadron which just so happens to be headed by Rico’s former high school teacher, Lt. Rasczak. Rico’s abilities impress Rasczak to the point of being promoted and, in the process, he finally comes around to reciprocating Dizzy’s desire.
Responding to a distress call on Planet P, they find an outpost utterly annihilated by the army of Bugs. What they do not find out until it is too late is that the call to the outpost is actually a trap and in the attack by the Bugs, Rasczak ultimately must be euthanized after the spiders bite off his legs. Then Dizzy takes a knife in the gut and ultimately dies in Rico’s arms just as they are rescued. One of those rescuers just so happens to be Carmen and Rico realizes once again that he is crazy about the girl.
Soon afterward, they learn of the possibility that there is some sort of more intellectually advanced and less savage “brain bug” that is guiding the attacks against the humans. Rico is handed full command of the Roughnecks with the plan to return to Planet P and get that bug with the big brain.
When the Bugs fire at the arriving fleet, Carmen’s ship is destroyed. Thankfully, she escapes in a pod. Less thankfully, the pod crash lands into a tunnel system. More thankfully, this tunnel system is proximate to Rico’s location. Unaware of what is happening, Rico is actually being guided by Carl whose work in Military Intelligence focuses on the use of psychic suggestion. Rico heads to the tunnel for the rescue.
Upon reaching the tunnel, Carmen and her cohort are utterly surrounded by soldier Bugs along with the Brain Bug. Zander, Carmen’s accomplice, gets his skull penetrated and his brain sucked out. Carmen is next in line. Carmen surprises the Brain Bug with a knife and slices off the proboscis that it uses for this sucking procedure. Meanwhile, Rico threatens to annihilate the Bugs by detonating a nuclear device. The brain inside the smart bug is developed enough to realize what Rico is suggesting and allows the captives to escape. Nevertheless, they are pursued by soldiers in the Bugs army and one of the members sacrifices himself for the greater good by detonating the nuclear bomb while the others make their escape to freedom.
Upon reaching the surface, they discover that the Brain Bug has been captured. Carl extends kudos to Rico and informs him and Carmen that the humans are on the verge of total victory now that Military Intelligence have unrestricted access to study of how the Big Brain operates. A psychic reading conducted by Carl is also promising: the Brain Bug is afraid. Very afraid.
The films ends with Carmen and Rico starring in a propaganda film calling for new enlistments in the ongoing struggle against the enemy.