After inheriting their parents' company and joining Ultraterrestrial Trouble Solutions, siblings Yuuhi and Yuamu Oudou began working to hunt aliens despite never encountering one before. Driven by his desire to meet an alien, Yuuhi believed that the mysterious theft of his "Earthdamar" was evidence of extraterrestrial activity. While searching for aliens in a forest, they discovered a crop circle-like formation where many trees had fallen. As they explored further, they stumbled upon a spaceship and entered it out of curiosity. To their astonishment, they encountered Yudias Velgear, an alien from the Velgear Star Cluster who was seeking to learn about Rush Duel-a popular card game among elementary school students-in hopes that it would save his home from war. With Yuuhi and Yuamu agreeing to teach Yudias about Rush Duel, they soon realized that their first encounter with an alien might not have been a mere coincidence.



