After taking over their parents' firm, brothers Yuuhi and Yuamu Oudou began working at the Alien Trouble Consultancy, a company dedicated to hunting extraterrestrials. Though they had never met an alien before, Yuuhi was eager to encounter one because he suspects that a creature stole his prized possession, the "Earthdama."
During a search for otherworldly beings, Yuuhi and Yuamu discovered an odd clearing in the woods where numerous trees lay toppled, creating a pattern reminiscent of a crop circle. Venturing deeper, they stumbled upon a mysterious spacecraft and entered it out of curiosity. Inside, they found Yuudias Velgear, an alien from the Velgear Star Cluster. It turned out that Yuudias wanted to learn about Rush Duel—a card game popular with elementary students—believing it could help save his homeland from a protracted war.
The siblings agreed to teach Yuudias Rush Duel, and soon realized that their first alien encounter might not have been mere chance. In the twilight, they sensed a faint hum that echoed through the forest, hinting at deeper mysteries.
