Taehyun Choi lives with his sister Hakyung Choi. You could say they have a normal life. On the one hand 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. Which is why 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.

(Source: MU)