From Chibi Manga:

Immorality and two-timing can't work with love. She was enjoying her job and she was hoping to go to heaven by herself... Somei Saya, 26 years old (unmarried) is that kind of woman. And then she'll bought an apartment at the mansion "Hitori-sama". She always thought that becoming an OL is the way to become a woman. that's why she was confident about her future... but then some problems happened! That's because of her colleague Kujo Aoi... he's too confident and a problematic person... and he's also the owner of her mansion...