The main character, Aoi Miyamae, is a male university student.

Aoi's twin sister, Riko Miyamae, has a different gender from Aoi, but they are identical in appearance and facial features.

Riko works as a clerk at 'LingerieS', a high-end women's lingerie specialty store. However, she wants to take two months off from her job to catch up with her favorite band on their national tour. However, she can't afford to miss work.

So, Riko asks Aoi, who looks exactly like her, to dress up as a woman and work at 'LingerieS' instead of her.

Of course, Aoi firmly refuses. However, Aoi reluctantly agrees to Riko's request.
Aoi enters a high-class women's lingerie specialty store for the first time in his life. He dresses as a woman and works as a clerk at the store.

While working as a clerk at 'LingerieS', Aoi discovers the depth of women's lingerie, interacts with the women who visit the store, and deepens his bond with them...

[From Steam]