"Full Metal Panic? Fumoffu" is a spin-off of the "Full Metal Panic!" series, focusing primarily on the comedic and slice-of-life aspects of the story. The series shifts away from the main plotline of mecha battles and geopolitical intrigue, instead centering on the daily lives of its characters in a high school setting. The protagonist, Sousuke Sagara, is a highly skilled but socially inept soldier from the covert anti-terrorist organization Mithril. He is assigned to protect Kaname Chidori, a high school girl who is believed to be a "Whispered," individuals with advanced technological knowledge embedded in their subconscious.

Sousuke's military background and lack of understanding of civilian life lead to frequent misunderstandings and over-the-top reactions to mundane situations. His attempts to blend in as a high school student often result in chaos, as he interprets ordinary events as potential threats and responds with excessive force or military tactics. Kaname, who is strong-willed and quick-tempered, frequently scolds Sousuke for his behavior, creating a dynamic of comedic tension between the two. Despite their clashes, their relationship gradually develops, with moments of genuine care and camaraderie.

The series is episodic, with each episode presenting a self-contained story that highlights the absurdity of Sousuke's military mindset in a civilian environment. One recurring element is Sousuke's use of a Bonta-kun mascot suit, nicknamed "Fumoffu," which he employs in various situations, often leading to humorous outcomes. The suit, originally intended for crowd control, becomes a symbol of Sousuke's awkward attempts to adapt to normal life.

Other characters include Sousuke's classmates, such as the mischievous Kyoko Tokiwa and the laid-back Shinji Kazama, who often find themselves caught up in Sousuke's antics. The series also features occasional appearances by Mithril operatives, who provide a connection to the broader "Full Metal Panic!" universe, though their roles are minimal and primarily serve to reinforce Sousuke's background.

The humor in "Full Metal Panic? Fumoffu" derives from the juxtaposition of Sousuke's military expertise with the trivialities of high school life. His exaggerated responses to everyday scenarios, combined with Kaname's exasperated reactions, create a comedic rhythm that drives the series. While the show occasionally touches on themes of friendship and personal growth, its primary focus remains on delivering lighthearted entertainment through its quirky characters and situational comedy. The series stands as a distinct departure from the more serious tone of the main "Full Metal Panic!" storyline, offering a comedic exploration of its characters' lives outside the battlefield.