Karai Tosou is a university student with a strong sense of justice, living an ordinary life until a masked group murders his entire family and leaves him for dead. He survives and is recruited by Shirotate Kotomi, a police inspector with ties to a secret organization called God Tools. God Tools distributes smartphones loaded with apps that grant supernatural abilities, turning users into either gods or devils. Karai is given one of these phones and appointed as a Criminal Hunter, tasked with hunting down the masked criminals responsible for his family's massacre.

The main conflict revolves around the Mask Guild, a syndicate of masked villains who use their own God Tools phones to commit heinous crimes. Karai's first target is Hedoro Kamen (Sludge Mask), the leader of the group that attacked his home. As he pursues revenge, he discovers that his father was a researcher for God Tools, working on advanced data that the masked criminals wanted. Karai also learns that he has been framed for Hedoro Kamen's crimes, forcing him to cooperate with God Tools to clear his name.

Early arcs introduce Masenuma Tamotsu, a bullied fellow student who is also a God Tools user, and the emergence of Devil Tools, a rogue faction of former God Tools operatives who now oppose the organization. Karai battles Pumpkin Mask, whose mask is a transforming drone that attacks with sharp shards, and Fox Mask, who wields a flamethrower gauntlet. After a brutal fight that leaves him injured, Karai receives a cyborg limb upgrade and a small AI assistant named Moka. Volume 2 culminates in Karai defeating Pumpkin Mask with a fiery punch and declaring his signature catchphrase, while the identity and motives of Masenuma remain ambiguous.