Storyline & Synopsis of Ubawareta Bokutachi

It follows the story of Sakai Yoichi, a freelance video director, and Mitsumi Kei, a piano teacher who moonlights as a serial killer. When Sakai Yoichi, who is stuck in his current situation doing small jobs to make ends meet, returns to his office, there's a package waiting for him. Inside is a human finger, and a message for Sakai Yoichi to get in touch if he's interested in "documenting his activities".

Sakai Yoichi was going to call the police but after considering, he decided instead to contact the sender. The person he meets at the designated house is none other than pianist Mitsumi Kei. This encounter leads Yoichi to continue documenting the murders but what awaits him as a result of this decision?