Johnny Rico, now a General, has been demoted and relocated to a Martian satellite station to train a new batch of recruits. The station is considered a remote and insignificant posting far from the front lines of the war against the Arachnids, a race of insect-like aliens that humanity is battling throughout the galaxy. Feeling sidelined and forgotten, Rico's frustration grows as he tries to impart his extensive knowledge and experience to a group of green recruits who lack discipline and are skeptical of his war tactics. His situation is complicated further by lingering issues back on Earth, where political maneuvering threatens to undermine the war effort.

Meanwhile, Mars is perceived as a non-strategic planet, and its inhabitants are not deemed worthy of heavy military protection. The Martian population is mostly focused on terraforming efforts, and the military presence there is minimal. Unexpectedly, the Arachnids launch a surprise attack on the planet, overwhelming the ill-prepared defenses. Rico and his inexperienced recruits are thrust into the chaos of battle, and he must quickly mold them into a fighting force capable of repelling the invasion. With the situation on Mars deteriorating rapidly, and Earth's support tied up in bureaucracy and politics, Rico and his team find themselves as the last line of defense against a relentless enemy aiming to overrun the planet.