Tatsumiya Village is located in the southernmost region of Japan. Time flows leisurely in a place where its residents have a strong affection for their hometown. In this village which has been with the sea for a long time, the villagers believe in mermaids, and the only shrine of the village is dedicated to them. With just a few days away from summer vacation, in the peaceful, yet lively daily life of Namishima Itsuki. The eldest daughter from Tatsumiya Shrine, giving an impression of a devout and dignified girl, Utsumi Hinowa. A friend who is the kindest of them all, despite being shy, aspiring to become a picture book author, Ogirihata Aoi. The three of them meet a sudden transfer student, a girl who becomes a new villager, Tachibana Aina. Since she comes from overseas, she is thrilled to see the village's scenery and customs for the first time. Her cheerfulness and positivity remind them of a girl they once knew, someone they had lost. One by one, the emotions that colored this brief moment are heated up like the sunshine of midsummer. Holding secrets and slight misunderstandings. Hoping and giving up, emotions and indifference. Spinning a tale of their summer memories. "What if", the assumption of it is meaningless, "It did not happen" is the only reality. In this short summer that passes by hastily, there is a story which gives a meaning to the past.