"Welcome to THE SPACE SHOW" is a science fiction anime film that centers on five elementary school children who embark on an extraordinary interstellar adventure. During their summer break, the group—comprising Natsuki, Kiyoshi, Noriko, Amane, and Koji—stumbles upon a mysterious, injured dog-like alien named Pochi while exploring the countryside. After nursing Pochi back to health, they discover that he is an alien researcher on a mission to find a rare plant called "wasurenagusa," which has the power to erase memories. In gratitude, Pochi invites the children to join him on a journey across the galaxy.

The children are transported to the moon, where they learn about an intergalactic society teeming with diverse alien species. They also discover that Pochi is part of a larger organization dedicated to studying and preserving the galaxy's resources. The group is introduced to advanced technology, including a spaceship powered by the wasurenagusa plant, and they set off on a quest to locate more of it. Along the way, they encounter various challenges, including hostile alien forces and bureaucratic obstacles within the galactic society.

As the adventure progresses, the children's bond deepens, and they each contribute their unique skills to overcome obstacles. Natsuki, the group's natural leader, demonstrates courage and resourcefulness. Kiyoshi, the youngest, provides comic relief but also shows unexpected bravery. Noriko, the most analytical, uses her intelligence to solve problems, while Amane, the quiet and observant one, often provides crucial insights. Koji, the most skeptical, gradually opens up to the wonders of the universe.

The narrative takes a darker turn when the group learns that the wasurenagusa plant is being exploited by a powerful corporation seeking to control its memory-altering properties. The children must confront the corporation's forces and protect the plant from misuse. Their journey culminates in a climactic battle that tests their resolve and unity. Ultimately, the children return to Earth, their memories of the adventure erased to protect the secrecy of the galactic society, but they retain a sense of the profound experiences they shared.

The film blends elements of adventure, humor, and science fiction, exploring themes of friendship, curiosity, and the ethical implications of advanced technology. The story is driven by the children's growth as they navigate unfamiliar worlds and face moral dilemmas, making it a compelling tale of discovery and resilience.