In the future, Earth is governed by a militaristic federation that promotes citizenship through military service, which grants rights not available to civilians. Johnny Rico, a young and idealistic high school graduate, joins the Mobile Infantry to earn his citizenship and to follow his high school sweetheart, Carmen Ibanez, who enlists in the Federal Service to become a starship pilot. Alongside his friends, Dizzy Flores, who harbors feelings for him, and Carl Jenkins, gifted with psychic abilities, Rico embarks on an intense boot camp training regime. Throughout their rigorous preparation, the group is indoctrinated with the values of the federation, and Rico rises through the ranks to become a squad leader. Meanwhile, Carmen thrives in pilot training, crossing paths with the charismatic flight instructor Zander Barcalow.

As the trainees progress through their respective military paths, humanity finds itself at war with an alien species known as the Arachnids, or "Bugs." The bugs are an insectoid race of aliens with fearsome fighting capabilities, and the conflict escalates quickly after an Arachnid attack on Earth claims millions of lives. In response, the federation mobilizes its forces for an offensive against the Arachnid home planet, Klendathu. The inexperienced Mobile Infantry recruits, including Rico and his unit, are thrust into the chaos and horror of all-out war. They must confront the brutality of the conflict and the stark reality that the propaganda and glamor of military service do not prepare them for the savage battles they must face against an overwhelming enemy.