In "SOS! Tokyo Metro Explorers: The Next," the narrative centers on a group of young adventurers who uncover a hidden world beneath Tokyo's subway system. The story begins when a mysterious earthquake disrupts the city, revealing an ancient underground labyrinth filled with advanced technology and remnants of a lost civilization. The protagonists, a diverse team of high school students, are drawn into this subterranean realm after discovering a cryptic message left by a previous explorer. Each member of the group possesses unique skills, ranging from engineering expertise to combat proficiency, which prove essential as they navigate the dangers of the underground.

The labyrinth is not uninhabited; it is populated by robotic guardians, environmental hazards, and rival factions seeking to exploit its secrets. The students must decipher clues, solve puzzles, and confront their own fears to progress deeper into the labyrinth. Along the way, they uncover the truth about the civilization that once thrived there, including its connection to modern Tokyo and the potential consequences of their discoveries. The narrative balances action, mystery, and character development, with each episode revealing new layers of the underground world and the group's evolving dynamics.

Key characters include the resourceful leader, a tech-savvy inventor, a cautious strategist, and a fiercely determined fighter, among others. Their interactions and personal growth are integral to the story, as they learn to trust one another and confront the moral dilemmas posed by their exploration. The series maintains a steady pace, blending episodic challenges with an overarching plot that builds toward a climactic revelation about the labyrinth's purpose and the fate of its creators. The story concludes with a resolution that ties together the characters' journeys and the mysteries of the underground, leaving room for potential future explorations.