When Mikoto suddenly wakes up, she has lost her memory and is being held captive with an unknown man. She was detained by six girls called Ranna, Fumika, Riri, Yu, Rumina, and Murumuru who barricaded themselves in this tower to escape from the zombies. Although suspicions remain, Mikoto is released from detention and allowed to stay in the tower. However, the next morning, the man who was with Mikoto was found dead. The culprit is among these seven people!?