Shinda Ishii no Taigun

Fourteen year old Yui Ishii wanted to die. She tried to kill herself many times, but now, for the first time, she wants to live. In a white room, 333 people who share the surname Ishii are gathered together. If they fail at the given task, an explosion will kill them instantly, and only one person can survive this death game.

At the Fushimi and Hachisuka detective agency, a call to help find the stage actor named Yuu Ishii. He was known for his unsettling performances, but he disappeared just before the final performance of his latest show.

What happened to Yui and Yuu, and why were the 333 people named Ishii chosen?

(Source: Kodansha, translated)