Dong ho always had a little crush on his upperclassman Ha neul. His friends convince him to ask for her number but he fails. the day gets worse when his father says he is going to remarry. They go to meet his soon to be step mother and step sister, only to find out it's his Ha neul noona. so what will happen when they are living together under the same roof, what will become of their new relationship as brother and sister?