After taking over their parents' business, siblings Yuuhi and Yuamu Oudou join Ultraterrestrial Trouble Solutions, an agency dedicated to alien hunting. Eager to retrieve his missing 'Earthdamar' from an alien, Yuuhi leads the search despite their lack of prior extraterrestrial encounters. During an exploration, they stumble upon a peculiar area in the woods resembling a crop circle, which leads them to a hidden spaceship. Upon entering, they meet Yudias Velgear, an alien from the Velgear Star Cluster. Yudias seeks to understand Rush Duel, a popular card game, to aid his war-torn home planet. As the Oudou siblings decide to teach Yudias the game, they realize this unexpected encounter holds deeper significance than they initially thought.