High school student Mitsusada Amaragi has a peculiar talent: he instinctively sees the positive side of everything, turning other people's insecurities into strengths. His classmates affectionately call him Brightside. One day, a mysterious app called Parashop installs itself on his phone, offering to sell peculiar superpowers. When he accidentally bumps into a stranger, the app purchases the ability to move just one piece of straw, and he is immediately attacked by another player—a cheerful but ruthless fighter named Saionji, who can control the impact force of rubber bands.

The game's rules are brutal: each player owns one ability at a time, and they can sell it back for points to buy a different power. By defeating opponents or finding clever uses for their ability, players raise its market value. If a player fails to accumulate enough points within a strict time limit, they die. After barely defeating Saionji, Mitsusada learns he has only 72 hours to reach 5,000 points—or face death. Reluctantly, he teams up with the now-friendly Saionji to survive.

Their first challenge is a frail old man, Yatagarasu Zen, whose nose-hair fire seems laughable but proves deadly. Mitsusada defeats him by exploiting his own straw to block air supply, winning enough points to stay alive. Next, they encounter Rio Onizuka, a delinquent girl who bought the ability to make any bread puffy and uses her fighting skill alongside it. After joining forces, they face the Hattori brothers, whose seemingly trivial abilities—making clothes rock-solid and rotating a part of a costume—become dangerous when combined. With quick thinking and teamwork, Rio and Mitsusada overcome them, raising his straw's value past the threshold.

Having survived the initial death game, Mitsusada, Saionji, and Rio reach the stage called Parashoppers, where the app announces the true competition has begun. They are joined by a mysterious priest, Kaoru Saito, whose ability to twist or manipulate threads makes him a powerful ally. Together they enter a new arena—a vast department store called Z, filled with more bizarre players and higher stakes. Each floor brings unique threats, such as the Pied Piper of Hamelin ability and a man named Goto with a hammer of dead spirits. Throughout these battles, Mitsusada's unshakable optimism and creative use of his mini-scale ability drive the group forward, while the group slowly uncovers hints that Parashop is controlled by an unknown entity watching their every move.