In the 23rd century, while colonizing new planets, humans have encountered a hostile non-technological insectoid species known as Arachnids, but commonly referred to as "Bugs". The Bugs appear to be little more than savage, unrelenting killing machines, though there are suggestions that they were provoked by the intrusion of humans into their habitats.
In the United Citizen Federation, citizenship is earned by performing activities such as military service, which grants individuals opportunities prohibited to basic civilians. After graduating from high school in Buenos Aires, John "Johnny" Rico, his girlfriend Carmen Ibanez, and psychic best friend Carl Jenkins enlist in the Federal Service, despite Rico's parents' disapproval of military service. Carmen becomes a spaceship pilot, while Carl joins Military Intelligence. Rico enlists in the Mobile Infantry and is surprised to find Isabelle "Dizzy" Flores, his fellow ex-student, has deliberately transferred to his squad.
In Mobile Infantry basic training, Career Sergeant Zim ruthlessly trains the recruits. Rico befriends fellow cadet Ace Levy and is later promoted to squad leader. He subsequently receives a Dear John letter from Carmen, as she desires a career with the fleet and now serves under Rico's high-school sports rival, Zander Barcalow. Following a live-fire training incident that kills one of Rico's squad members and causes another to quit out of guilt, Rico is demoted and flogged. He resigns and calls his parents to ask them if he can return home, but rescinds his resignation after an asteroid, reported to be launched by the Arachnids, obliterates Buenos Aires, killing his parents and millions of others.
An invasion force is deployed to Klendathu, the Arachnids' home planet, but the operation severely underestimates the Arachnids and is a total disaster. Rico is severely wounded and mistakenly reported KIA. After recovering, he, Ace, and Dizzy are reassigned to the "Roughnecks", an elite unit commanded by Lt. Jean Rasczak, Rico's former high-school teacher. He quickly gains the respect of his peers and is promoted to the rank of Corporal after taking out a tanker Bug. His relationship with Dizzy continues to grow, and they have sex during their night on Tango Urilla.
The Roughnecks respond to a distress call from Planet "P", where they reconnoiter an outpost that has been devastated by Bugs. They soon realize that the distress call was a trap, and the Arachnids swarm the outpost. Rico, now an acting sergeant, euthanizes a mortally wounded Rasczak after a buried tanker Bug bites off his legs. Dizzy is killed, but the surviving Roughnecks are rescued by Carmen and Zander. Rico and Carmen encounter Carl, now a high-ranking intelligence officer, at Dizzy's funeral. Carl reveals that there is reason to believe an intelligent "Brain Bug" is directing the other Bugs and has been learning how to fight humans. He field-promotes Rico to lieutenant and gives him command of the Roughnecks, ordering the Mobile Infantry units under his control to return to "P" in an attempt to capture the Brain Bug.
The fleet encounters unexpected heavy fire from the Bugs and Carmen's ship is destroyed. Carmen and Zander's escape pod crashes into a Bug tunnel system near Rico. They are surrounded by Bugs, and a Brain Bug uses its proboscis to pierce Zander's skull and eat his brain. As it is about to do the same to Carmen, she cuts off its proboscis with a knife. Rico, Watkins and Ace arrive and threaten the Bugs with a small nuclear bomb, which the Brain Bug recognizes. They flee while the Brain Bug makes its escape. Arachnids pursue them and Watkins, mortally wounded, sacrifices himself by detonating the bomb to enable the others to escape.
After returning to the surface, they find that former Sergeant Zim, who had requested a demotion to private so that he could serve at the front, has captured the Brain Bug. Carl tells Rico and Carmen that the humans will soon be victorious now that Military Intelligence can study the Brain Bug. Carl mentally scans the Bug and reveals that it is afraid, to the cheers of the troops. A propaganda clip shows Carmen, Ace, and Rico as model servicemen, encouraging viewers to enlist in the armed forces.
Casper Van Dien as Private / Corporal / Sergeant / Lieutenant John "Johnny" Rico
Dina Meyer as Private Isabelle "Dizzy" Flores
Denise Richards as Lieutenant / Captain Carmen Ibanez
Jake Busey as Private Ace Levy
Neil Patrick Harris as Colonel Carl Jenkins
Patrick Muldoon as Lieutenant Zander Barcalow
Clancy Brown as Career Sergeant / Private Zim
Michael Ironside as Lieutenant Jean Rasczak
Seth Gilliam as Corporal Sugar Watkins
Bruce Gray as Sky Marshal Dienes
Marshall Bell as General Owen
Matt Levin as Private Kitten Smith
Eric Bruskotter as Private Breckinridge
Brenda Strong as Captain Deladier
Christopher Curry as Bill Rico
Lenore Kasdorf as Mrs. Rico
Denise Dowse as Sky Marshal Meru
Amy Smart as Pilot Cadet/Lt. Lumbreiser
Dean Norris as Commanding officer
Rue McClanahan as Biology teacher
Dale Dye as Mobile Infantry general
Anthony Ruivivar as Shujumi
Robert David Hall as Recruiter
based on Robert A. Heinlein's 1959 novel of the same name.
The only theatrically released film in the Starship Troopers film series