16-year-old Nanakusa is spending his mundane life on "Stairway Island" where he found himself "discarded" along with 2000 other people, all missing the memories of the time just before they arrived there. The name of the island originates from the long stairway running through the island and extending up to the summit of the mountain where the witch who rules the island is said to live. The only way to leave the island is by finding what was lost. Yet Nanakusa doesn't wish to leave this stagnant island. He enjoys his life there.

That is until Manabe Yuu, his childhood friend appears in front of him, and turns his new life upside-down. Why did Manabe Yuu, the world's most optimistic person, get discarded, and who is the witch behind all these mysterious occurrences? Follow Nanakusa and Manabe as they uncover the true nature of Stairway Island.

Based on the bestselling youth mystery novel by Yutaka Kono and illustrated by Ai Uzuki.