Marielle is a young girl currently living in a remote house deep in the mountains between Switzerland and France to recover her health. The story is told through the memories of her adventures with Memoru, a tiny alien girl who lives on a nearby island. Her people are gnome-like beings who are stuck on Earth after their spaceship crashed here.

A ceramic figure of a blue mouse with long ears, stubby legs, and no tail suddenly comes to life. He is a “Manxmouse” as known in the legend. Since he is new to this world, his fearless curiosity leads him to meet a frog couple, an aviator hawk, a stray tiger, and a girl. Although he learns about his ultimate fate-to be eaten by a “Manxcat”-he decides to face this cat and challenge his destiny.

When local teacher Midori Yamabuki receives an emergency summons to her home planet it turns out to be a trap devised by galactic warlord Dr. Mashirito, and it's up to Dr. Senbei Norimaki and a group of students (including his robot daughter Arale) to venture into space to save her.