The main character's mother had died recently and he decided to go to his mother's hometown, a mysterious and isolated island that he and his mother had never returned/visited before, to tell his uncle who he had never seen before about his mother's death.
Only after arriving the island he found out that his uncle (and thus his family) was the lord of the island, and he was asked to stay on the island to succeed the title. When the main character was still pondering his decision, people on the island warned him about the full moon night and asked him to stay in the house that day, but of course he ran out in order to find Sadaharu, his life saver and friend, and found out that people on the island went all insane.
The story so far has no important connection with your supposed lovers, who are: (sorry I'm not too sure about how to pronounce their kanji)
辰馬 (Tatsuma?), a rugby man from the same university of the main character that dropped on rail trail and was later saved by the main character and Sadaharu. He is from the shrine on the island and used to have superpower that he cannot use anymore.
定晴 (Sadaharu), a fisher who saved the main character and Tatsuma when they were about to be hit by a train. His purpose of visiting the island is not yet known.
巖 (Iwashi?), a fierce and often angry man that travels with Youichi.
洋一 (Youichi), a silent man that seems to be overly tolerant of Iwashi's bad behaviors (and pretty much everything else).
千波 (Sena), a young fisher living on the island. He seems to have some curse that people around him would act strangely like they do in a full moon night.