It has been almost a year since Mikado Ryuugamine moved to Ikebukuro, and the city seems stagnant. He recalls Izaya Orihara's words: even the most eventful city like Tokyo will eventually feel monotonous. However, when Mikado's old friend Miyoshi Yoshimune returns to Raira Academy, life picks up its restless pace once more.

Meanwhile, during the calm, Shizuo Heiwajima wishes more control over his anger outbursts. His boss, Tom Tanaka, suggests they go see the new popular movie starring Shizuo's brother Kasuka, the film which has brought overwhelming popularity to the attaché cases everyone now seems to be carrying. But despite the apparent peace, Ikebukuro's anonymous online chat—now joined by Yoshimune—talks about an unsettling development: attaché case thefts are on the rise and odd tornadoes are hitting Ikebukuro—yet no one can understand why.