It's the hottest summer in over three decades in South Korea, and Kwon Yuri has persuaded her best friends Im Yoona and Seo Joohyun (Seohyun) to spend their vacation as lifeguards at Caribbean Bay, an indoor/outdoor waterpark. Yoona is reluctant - she has enough responsibilty as it is, paying tution bills and apartment rent - but when she meets Ok Taecyeon, a cute and flirty lifeguard, she's happy that she has joined the staff at Caribbean Bay. Or atleast she thinks she is. Taecyeon's "old friend" Hwang Miyoung (Tiffany) keeps showing up at all the wrong times. Nichkhun Horvejkul and Hwang Chansung, her co-lifeguards, do everything in their power to make her feel like an outcast. And her bills still aren't getting paid. Will Yoona ever adjust to her new lifestyle?