Taehyun Choi lives with his sister Hakyung Choi. You could say they have a normal life.
Hakyung has a good job and is successful, however Taehyun isn't good at studying.

His life takes a drastic change when he accidentally learns that he and Hakyung aren't siblings.
Thus he decides to confront her, but then she suffers a serious accident and loses her memory.

This is where everything will change, because those feelings of kinship will become much more important.