As Kim Jongdae half-sister, Kim Yeonhee knew that her brother doesn't really comfortable with her presence. Their already drifted relationship becoming more further as the girl also behaving like her brother as she grew older. Jongdae's friends also learnt that situation long ago and doesn't have a nerve to hold a conversation with her because they knew Jongdae's will not like it. It's not like Jongdae really hate his sister but he cannot accept that his father married to another woman after the death of his mother, so he expressed his anger toward his half-sister. However, after witnessing his sister bright smile when she was having fun with her friends, he came to a realisation that she never smiled at home and always wear her poker face. Whenever his friends came, she will either locked herself in her room or immediately going out, not wanting to encounter to any of his friends. 

What will happened, when Kim Yeonhee was entangled in a situation which she never expected to happen with one of her brother's friend, Byun Baekhyun and how will Jongdae reacted upon the surfaced problem between his friend and his sister that he never grew fond with?