Siyeon, a top secret agent from the country's top secret organisation, met Kyungwon through Minkyung. Kyungwon and Minkyung are very close friends working in Seoul Hospital. Minkyung's dad is the head of the organisation and has been taking care of Siyeon ever since her parents both passed away. Therefore, Minkyung always considered herself as Siyeon's big sister.And only Minkyung and her parents knows about Siyeon's job excluding those who are in the organisation. After the first time Siyeon