Summer Pockets: Summer's Treasure

"It's my first time coming here, yet why does it feel so nostalgic?" Hairi, a high school student, arrives at Torishirojima to take care of his late grandmother's belongings. As he wanders around the island, he meets a mysterious girl, Shiroha, swimming in a pool in the middle of the night. Then, the next morning, he meets another young girl, Umi, who is his relative?