"Matchless Raijin-Oh" is a mecha anime series set in a near-future Earth where humanity faces threats from an alien race known as the Jyaku. The story centers around a group of elementary school students who form the Earth Defense Class, a secret organization tasked with protecting the planet using advanced robotic technology. The protagonist, Daisaku Nishikawa, is a fifth-grader who pilots the titular Raijin-Oh, a powerful robot formed by combining three smaller machines: Raijin, Fujin, and Suijin. Alongside his classmates, Daisaku battles the Jyaku, who frequently send monstrous creatures called Jyakin to attack Earth.

The Earth Defense Class operates from their school, which doubles as a high-tech base equipped with advanced weaponry and defense systems. The students balance their daily lives as ordinary children with their responsibilities as defenders of Earth, often facing challenges that test their teamwork and resolve. The Jyaku, led by the enigmatic Emperor Zuhl, seek to conquer Earth and exploit its resources, leading to frequent confrontations with the Earth Defense Class.

The series emphasizes themes of friendship, courage, and perseverance as the young protagonists grow both as individuals and as a team. Each episode typically features a new Jyakin threat, requiring the students to devise creative strategies to defeat their enemies. The Raijin-Oh robot is central to these battles, with its combination sequences and powerful attacks serving as key elements of the action. Supporting characters, such as the resourceful teacher Ms. Midori and the intelligent classmate Kenji, play vital roles in assisting the team and advancing the plot.

As the series progresses, the stakes escalate, with the Jyaku launching increasingly dangerous attacks and revealing deeper layers of their plans. The Earth Defense Class faces personal and collective challenges, including moments of doubt and conflict, but their unity and determination ultimately drive them to protect their world. The narrative balances episodic battles with overarching storylines, culminating in a climactic confrontation with the Jyaku that tests the limits of the students' abilities and resolve.