There is a macabre story circulating throughout the halls of Akanegasaki Middle School: unsuspecting teenage girls will hear the sound of a bell when they are walking alone at night, and suddenly, a woman in a long coat will appear. Known as the Ripper, the story goes that she will ask their name and then stab them repeatedly after they answer.

Haruka Kanade, Arisa Narumi, Chisato Shion, and Matsuri Hinata have all heard the rumors, but they know full well that the stories stem from actual murders taking place in a neighboring city. And unlike most, the girls feel confident that they can do something to stop it, as each of them have contracted with a cat-like creature named Kyuubey to become magical girls. As they are patrolling one night, they stumble upon the so-called Ripper but are left shaken by the encounter—after all, the killer is also a magical girl.