Jinyoung's sister is head-over-heels for this one senior member of the school's swim club, Mark. She's a bit shy so she seeks moral support from her brother and convinces him to join the club as well. But things don't go like they expected and a variety of problems arise: one, Mark is not only hot as hell, but also very kind, which intrigues Jinyoung; two, Jinyoung apparently at swimming; and three, Mark takes good care of the newbies and offers extra help (and Jinyoung needs all the help he can get), and ends up giving Jinyoung a lot more attention than his sister. (Prompt SD-05)