In the 23rd century,^[i] Earth is governed by the United Citizen Federation, a stratocratic regime founded generations earlier by “veterans” after democracy and social scientists brought civilization to the brink of ruin. Citizenship is exclusively earned through federal service, which grants rights—like voting and procreation—that are withheld from ordinary civilians. Humans, who are now spacefaring, conduct colonization missions throughout the galaxy, bringing them into conflict with a race of highly evolved insectoid creatures dubbed “Arachnids” or, derisively, “bugs”.

Against his parents’ objections, teenage jock Johnny Rico enlists as a mobile infantryman to remain close to his girlfriend, spaceship pilot Carmen Ibanez. Their psychic friend Carl Jenkins joins Military Intelligence, while Isabelle “Dizzy” Flores, who is in love with Rico, deliberately transfers to his squad. Carmen eventually ends her relationship with Rico due to their diverging career paths and her growing feelings for fellow pilot Zander Barcalow. During training, Rico impresses drill sergeant Zim, earning a promotion to squad leader. However, a mistake during a training exercise leads to the death of a squad member and the resignation of another, resulting in Rico’s demotion and flogging. Disheartened, Rico quits but re-enlists after learning that an asteroid sent by the Arachnids has destroyed Buenos Aires, killing millions, including his parents.

An invasion force is deployed to Klendathu, the Arachnids’ home planet, but military intelligence underestimates the Arachnids’ defensive abilities, leading to thousands of casualties. Badly wounded, Rico is rescued by lieutenant Jean Rasczak, his former high-school teacher, but is mistakenly reported dead, devastating Carmen. Following his recovery, Rico, Dizzy, and squadmate Ace Levy join Rasczak’s elite unit, the “Roughnecks”. Rico’s valor in battle earns him the rank of Corporal, particularly after he defeats a gigantic “Tanker Bug” on Tango Urilla, and he develops a romantic relationship with Dizzy. Responding to a distress signal on Planet P, the Roughnecks discover an Arachnid-ravaged outpost and are ambushed by the bugs. Carmen and Zander recover the surviving Roughnecks by dropship, but not before Dizzy is mortally wounded and Rico mercy kills the mutilated Rasczak. The group returns to the fleet assembled in orbit above P, where Dizzy is eulogized.

Jenkins, now a high-ranking colonel, reveals the Roughnecks were deliberately ordered into the trap, justifying it as a necessary sacrifice to confirm the existence of a “Brain Bug”, an intelligent Arachnid strategically directing the others. He assigns Rico command of the Roughnecks and field-promotes him to lieutenant, instructing him to return to P and capture the Brain Bug. As the battle commences, Carmen’s ship is destroyed by the Arachnids, and the escape pod carrying her and Zander crashes into an underground tunnel system. The pair are captured by the Arachnids and the Brain Bug consumes Zander’s brain, absorbing his knowledge. Rico directs his squad to complete their mission while he, Ace, and their squadmate Watkins rescue Carmen and hold the Arachnids at bay with a miniature nuclear bomb.

The Brain Bug escapes while the Arachnids attack and fatally wound Watkins, who sacrifices himself by detonating the bomb while his teammates escape. On the surface, they learn that Zim has captured the Brain Bug, and the assembled troops rejoice as Jenkins psychically detects it is afraid. A propaganda broadcast details how the Brain Bug is being invasively studied to learn its secrets and ensure humanity’s victory. The advert encourages viewers to enlist and do their part in the war so they can become like Carmen, now captain of her own ship, and Rico, who enthusiastically leads his troops into another battle.