When the protagonist and his girlfriend, Hotaruko, had sex for the first time, they both simultaneously saw a strange image in their minds. When they tried investigating it, it led them them to a solitary island - Amatojima.

Thinking that it's somehow related with Hotaruko's lost memories, they decided to visit the island during their summer break. After a one day trip from Tokyo, they arrived at Amatojima. They were the only passengers getting off the boat there, and the harbor was surprisingly deserted. They went to the only lodging house to book a room for the night. When the old lady who owned it heard Hotaruko's surname, her expression suddenly changed. She told them that Hotaruko is related with the island chief, and hurriedly directed them to the Kizuki household.

There, they were welcomed by a woman called Kasumi, who led them to the guest room. The island chief, Kizuki Mizuno-sama, despite his suspicious appearance, gave them a very warm reception. It was decided they will stay at the Kizuki household for a little while...

[edited from the official website]