An anonymous chat group comprised of the quirkiest residents in Ikebukuro gets an unexpected member – Saika. This cryptic user sends perplexing messages and is often dismissed as spam. However, the group becomes intrigued when they realize that Saika's messages may be connected to an ongoing crime spree of slashing attacks terrorizing Ikebukuro. The media has dubbed the assailant the "Slasher" and their reign of terror continues.
In the midst of all this chaos, Anri Sonohara is warned to keep her distance from a teacher with a history of dating minors. Despite having friends, Anri still faces bullying, and during one instance, she unexpectedly encounters the Slasher. Nonetheless, she miraculously survives the encounter unscathed.
Concurrently, a reporter investigating the "strongest man in Ikebukuro" is savagely assaulted by the Slasher. Shizuo Heiwajima, a front-runner for this title, takes it upon himself to track down the person responsible. It is then revealed that the Slasher has potential supernatural abilities which attract the attention of infamous informant, Izaya Orihara. Ikebukuro may face its next great otherworldly threat.