"Sgt. Frog" centers on Keroro, a bumbling yet ambitious alien frog from the planet Keron, who leads a platoon of frog-like soldiers tasked with invading Earth. However, after being discovered by the Hinata family—consisting of siblings Fuyuki and Natsumi, along with their mother Aki—Keroro is forced to abandon his mission and live with them as a houseguest. Stripped of his weapons and resources, Keroro spends his days indulging in hobbies like building Gundam models and watching anime, often neglecting his original goal of world domination.

Keroro’s platoon, known as the Keroro Platoon, includes four other members: the stoic and sword-wielding Giroro, the tech-savvy and introverted Tamama, the enigmatic and artistic Kururu, and the laid-back and gluttonous Dororo. Each member has distinct personalities and skills, but their collective incompetence and frequent infighting often hinder their plans. Despite their alien origins, they integrate into human society, forming relationships with the Hinata family and other Earth residents, including the wealthy and eccentric Momoka Nishizawa and her robotic maid, Angol Mois.

The series blends episodic comedy with occasional overarching plotlines, often revolving around the platoon’s attempts to conquer Earth, which are consistently thwarted by their own ineptitude or the intervention of the Hinata siblings. Natsumi, in particular, frequently takes on a protective role, using her martial arts skills to keep Keroro and his team in line. Fuyuki, on the other hand, is fascinated by the supernatural and often aids the platoon in their schemes, albeit unintentionally.

Throughout the series, the Keroro Platoon encounters various adversaries, including rival alien factions, Earth’s defense forces, and even their own superiors from Keron. Despite their antagonistic origins, the platoon gradually develops a sense of camaraderie with the humans, blurring the lines between their mission and their newfound friendships. The narrative explores themes of loyalty, identity, and the absurdity of war, all while maintaining a lighthearted and comedic tone.

The humor in "Sgt. Frog" is heavily reliant on parody, referencing popular anime, manga, and Japanese pop culture, making it particularly engaging for audiences familiar with these elements. The series also incorporates moments of heartfelt emotion, particularly in episodes focusing on the platoon’s backstories or their evolving relationships with the Hinata family. Overall, "Sgt. Frog" presents a unique blend of sci-fi, comedy, and slice-of-life elements, anchored by its quirky characters and their misadventures on Earth.