High schooler Yuri Honjo wakes up in a nightmare world of skyscrapers linked by narrow suspension bridges, with no ground below and no one to explain how she got there. Stalking the rooftops are the Masks, figures in expressionless masks who hunt the trapped and push them to jump or kill them outright. As Yuri fights to survive and searches for her brother Rika, she pieces together the rules of this sealed world and the killers who enforce them.