Summer Pockets: Kushima Kamome-hen

Theatrical edited version of Summer Pockets following Kamome Kushima's storyline from the anime series.
Kamome, who once visited Torishiro Island as a child reveals that her purpose in coming to the island is to "search for treasure", and thus sets off on an adventure with Hairi.
What is the treasure hidden on the island? And what is the past that connects the two of them?
(Source: TOHO Cinemas, translated)