"Nezumi no Densha～Nanatsu Ko-Nezumi Series 1" is an anime series that revolves around a group of seven young mice who embark on a journey aboard a mysterious train. The narrative is set in a world where anthropomorphic mice coexist with humans, and the train serves as a central element that connects various adventures and challenges faced by the protagonists. Each of the seven mice possesses distinct personalities and skills, which they utilize to overcome obstacles and solve problems encountered during their travels.

The story begins with the mice boarding the train, which is revealed to be no ordinary vehicle but a magical entity capable of traversing different realms and timelines. The train's destination is unknown, and its purpose becomes a recurring mystery throughout the series. As the mice journey through diverse landscapes and encounter various characters, they uncover fragments of a larger puzzle tied to the train's origins and its connection to their own lives.

Key characters include the resourceful leader, the cautious strategist, the adventurous daredevil, the empathetic mediator, the curious inventor, the artistic dreamer, and the pragmatic realist. Their interactions and growth form the emotional core of the series, as they learn to rely on one another and confront their individual fears and insecurities. The episodic structure of the narrative allows for self-contained adventures while gradually building an overarching plotline that ties the characters' pasts to the train's enigmatic nature.

Themes of friendship, perseverance, and self-discovery are explored as the mice navigate challenges ranging from external threats to internal conflicts. The series balances lighthearted moments with more serious undertones, creating a dynamic tone that appeals to a wide audience. The train itself becomes a symbol of transformation and exploration, reflecting the characters' evolving understanding of themselves and their world.

As the series progresses, the mice uncover clues about the train's true purpose and its connection to a forgotten legend involving their ancestors. This revelation sets the stage for a climactic confrontation that tests their unity and resolve. The narrative concludes with a resolution that ties together the various threads of the story while leaving room for further exploration in potential sequels or spin-offs.