Second-year high school student Miharu Seno has always coasted through life on easy mode. He is academically gifted, good-looking, and consistently beats his friend Kenya Fujishiro at video games. One day at the arcade, a teddy bear-like creature steals his wallet. Chasing it into an abandoned storage room, Miharu encounters a flamboyantly dressed man who climbs out from inside an arcade screen. The man calls himself Romeo and claims to be from the future. He explains that Miharu has been chosen to partner with him in the D-Game, a deadly competition with vague stakes and a severe penalty for losers: death. The teddy bear, Edward, serves as their Judge, also from the future. Miharu, unwilling to forfeit by default, agrees to participate.

The D-Game consists of a series of challenges that pit Miharu and Romeo against other player-and-judge pairs. The bright, playful appearance of the judges and the arcade-like nature of the games contrast sharply with the lethal consequences of losing. As Miharu and Romeo progress, they face a variety of one-time games that test their wits, cooperation, and resolve. Along the way, they meet other competitors, including the mysterious Shirou Asuma, who becomes a supporting figure. The narrative builds toward a final, large-scale game where all remaining players are brought together. In the climax, the true purpose of the D-Game and the nature of the future from which Romeo and Edward came are revealed in a rushed and ambiguous resolution. Only Miharu and Romeo receive definitive arcs and closure, while the surrounding cast is left largely unexplored.