Minase ── an isolated island due to its poor accessibility. However, the protagonist used to go there a lot in his childhood.
After entering medical school, his parents were worried about him losing his edge due to the daily study schedule. They wanted to refresh him. The protagonist comes to the island again after being advised to visit his cousins during his summer vacation. He remembers his childhood with them, and being treated like a real family member.
Suiren was beautiful when she was a child, and she was very kind and was like a real sister to him. Shion was like a tomboy little sister and used to follow him around.
The protagonist wonders how the two of them have grown up. He can hear their voices afar from their house. Upon his arrival, he reunited with the two grown-up sisters.
Suiren has become a gorgeous woman with a beautiful feminine body overflowing with tolerance and kindness. Shion, who has grown into a pretty girl, has a somewhat disgruntled look and only talks to the bare minimum necessary.
An older sister who never changes and a younger sister who has changed. Feeling puzzled, as recommended by Suiren, who seems to have been waiting for the main character, the protagonist stay at their house for a while.
How will he spend his time with them on the island under the summer sun?