The novel centers on the experiences of the members of a military force that operates in space. The protagonist is Juan Rico, nicknamed “Johnny,” a member of the Mobile Infantry’s elite squad, Rasczack’s Roughnecks. Some of the other characters are his friends and family before he became a trooper, and the others are his service members. They fight the enemies of Earth, governed by the Terran Foundation. The aliens are the Arachnids or Bugs.

Johnny comes from a well-to-do family, but his father rejected the son’s patriotic desires to join up. Johnny went ahead anyway, accompanying his friend and hoping to impress a girl. The selection process sent him to MI, however, where he proved adept at the demanding tasks and was appointed to the toughest squad; Johnny becomes an officer and further distinguishes himself, commanding a raid on the Bug elites. In recognition of his heroism, the unit is renamed “Rico’s Roughnecks.” During the course of his training and early assignments, the young man learns that his mother died in a bombing and his father had a change of heart and also joined the military.

Johnny’s parents do not support him in his interests and efforts. His father, a wealthy businessman, denigrates civic-mindedness, especially opposing the Federal Service requirement, and promotes capitalism as independent of patriotism. Later, he changes his point of view, enlists as a private, and is soon promoted to sergeant. Johnny’s mother tries to run his life and becomes angry when he enlists. She later dies when the Bugs bomb Buenos Aires.

Carl is Johnny’s childhood friend with whom he enlists. Assigned to a research facility, he is killed when the Bugs attack it.

Carmen is their classmate on whom Johnny has a crush. Aided by her excellence at mathematics, she later becomes a pilot (one of the careers into which women are tracked).

Sergeant Charles Zim is Johnny’s drill instructor during his training. He had gone through basic training with Jean Dubois, Johnny’s high school civics teacher.

Lieutenant Rasczack, for whom the unit is named, dies in a battle. Sergeant Zelal, one of Johnny’s his first superiors as a roughneck, recommends that he enter officer training. He later loses his legs and becomes “bionic” through his new prosthetic legs.