He thinks she's just a maid. 

Oh Sehun is a filthy rich 18 year old with dozens of maids and servants in his ultra huge mansion. 

He doesn't think much about all the maids and servants, but who does? 

They're MAIDs and SERVANTs. Nothing important. 

She knows the truth. 

One of the maids working for the Oh family has a secret. 

Her real name is Oh Seeun, but she goes by the name of Kim Heesun. 

What's her secret? 

She's the missing daughter of the Oh family.