High school student Nanakusa and 2,000 other people find themselves abandoned on an island called "Stairway Island", and everyone has lost all their memories before arriving on the island. There is a long staircase on the Stairway Island that runs through the island and extends to the mountain. The witch who rules the island is said to live on this mountain. The only way to leave the island is to find out what he has lost. However, Nanakusa does not want to leave here, he likes life here. Until one day, Nanakusa's childhood playmate, Manbe Yuu, appeared and disrupted his life. Why would a cheerful person like Manbe Yuu be abandoned? Who is the witch behind these mysterious events? The two began to explore the truth of Stairway Island step by step...