The game does contain mature content, but the Steam version is labeled as the all ages release. However, MangaGamer is offering the non-mosaic mature version on their site for %10 off before that game’s release.
The publisher details the premise:
As the eldest son of the Katsuragi Family — one of the leading family of mages in Japan — Kiyotaka travels across the ocean to London with his little sister, Himeno, in order to study abroad at Royal London Magic Academy, also known as Weather Vane.
A sprawling underground academic city awaits Kiyotaka and Himeno as they set foot for the first time in the fog-covered city of London. The academic city is situated in an underground lake and is composed of three floating islands in the shape of a crescent moon.
Many fledgling mages from around the world congregate here to study their craft in an optimum environment. Kiyotaka begins to feel at ease in his new environment, and soon enough the siblings are merrily hanging out with their new friends at school. Experiencing ordinary days, repeating like a da capo, as if they were a dream…
Kiyotaka walks through London together with Ricca one day. The two meet a lone little girl, situated right underneath a giant clock tower. The girl carries a small cherry blossom branch in her hand, and is missing most of her memories. They decide to take care of her at Weather Vane for a while after learning that she possesses magical powers.
But Kiyotaka had the wish entrusted to him by his father. In order to realize his goal, he needs to become a member of Weather Vane’s student council. So for that reason, Kiyotaka nominates himself to run in the student council elections. Meanwhile, their class receives missions from The Queen. There are those who would use the power of magic to hurt others—Kiyotaka and his classmates head into foggy London in order to prevent those incidents before they occur.
A fog that is present all year, a mysterious girl with amnesia, and the magic cherry blossom branch she held. The gears of Kiyotaka’s destiny begin to turn. A tale of encounters and farewells, repeating like a da capo.
You can watch the opening movie as well as some semi NSFW CG images below: