The old anonymous group chat consisting of some of Ikebukuro's most peculiar residents receives a sudden addition: Saika, a cryptic user who occasionally pops up and sends messages that make little sense. Although everyone simply discards it as spam, a curious connection between Saika's messages and a recent string of slashing assaults soon forms.

Dubbed the "Slasher" by the media, their rampage continues. At the same time, Anri Sonohara is advised to stay away from a teacher with a history of dating minors. Although she now has friends, Anri still suffers from bullying; during one incident, she comes face-to-face with the Slasher, becoming the first person to survive the encounter unscathed.

Meanwhile, a reporter following a story about the so-called strongest man in Ikebukuro is viciously attacked by the Slasher. As the number one contender for the title of strongest man, Shizuo Heiwajima sets out on a hunt to find the mysterious aggressor. In his latest uncovering, the infamous information broker Izaya Orihara reveals that the Slasher might just be Ikebukuro's next supernatural threat.