Dai Yagashira appears to be an ordinary wealthy high school student, but he is secretly a member of the International Treasure Hunters Association (ITHA) and the heir to a long line of treasure hunters. Together with his classmate Yuki Dazai, he travels the globe collecting rare and supernatural objects known as Alien Treasures – artifacts of extraterrestrial or otherworldly origin that possess extraordinary powers. Their hunts pit them against military forces, private militias, and rival treasure hunters, all while navigating a web of international conspiracy and hidden knowledge.

The series blends action, science fiction, and mystery, with each volume forming a self-contained adventure that also feeds into a larger narrative. Early arcs such as Alien-Hidden Treasure Town and Alien-Monster Zone introduce the core mechanics of the treasure hunts and the various factions that oppose or ally with Dai. Later arcs grow more ambitious: Alien-Apocalypse Record delves into apocalyptic threats, while the multi-volume Alien-God’s Country and Alien-Black Death Empire explore the origins of the alien treasures and the true scope of the powers at play. Throughout the series, Dai relies on a mix of high-tech gadgets, ancient artifacts, and his own combat skills to survive encounters with supernatural creatures, rogue agents, and ancient conspiracies.

The setting shifts across real-world locations, from hidden towns and mystical mountains to treacherous seas and underground empires. The story is driven by Dai’s determination to unearth the secrets of the Alien Treasures and protect them from falling into the wrong hands, all while maintaining his cover as a normal teenager.