Getting out of the house can be a problem, especially when you're part of the country's biggest idol group, ZoNE. Jessica, Sooyoung, and Yoona were finally given a day to relax when Jessica and a strange, feverish girl named Taeyeon crossed paths. In an odd turn of events, Taeyeon remains at the ZoNE dorm and makes a strange bet with Jessica: if Taeyeon can get someone she asks out to fall for her, Jessica wins. Low and behold, Taeyeon has her eyes on Jessica, in more ways than one. Taeyeon's past starts to peek out as Sooyoung and Yoona's protective sides try to stand between Taeyeon and Jessica, and strange events start to occur, culminating in the revelation of who Taeyeon really is, who some girl named "Krystal" is, and how she is was sent by someone to kill Jessica. Taeyeon promises she won't kill Jessica, who decides to trust her against Sooyoung and Yoona's judgments. Taeyeon announces that others will be after Jessica, so she will be her bodyguard.
Seohyun, the first assailant, attempts to kill Jessica. Her angered heart that still harbored envious feelings towards Jessica drove her to a corner where a masked stranger offered help in exchange for Seohyun's services. Taeyeon and Jessica reasoned with Seohyun and got away safely.
Hyoyeon, a spy for Taeyeon's ex-boss, tries to get Taeyeon to come back to her side by blackmailing her and her friends Sunny and Tiffany. But with the help of Eunhyuk, Hyoyeon's crush, Taeyeon is able to help Hyoyeon out of the dark and recruit Hyoyeon as a new ally.
Yuri, the next attacker, was right by Jessica's side all along. As a trainee under the same company, she had every chance she wanted to kill Jessica. She just didn't want her hands to get dirty. Yuri's secret slips and a dangerous fight ensues. In the end, Yuri is killed without ever saying her true motive.
Taeyeon, Jessica and their friends think all will be over if they can trap Taeyeon's ex-boss during a live recording. But Taeyeon ends up being the one caught in a trap and is taken far away.

With some smart thinking on Hyoyeon, Eunhyuk and Sunny's part, and Seohyun's additional help, the four make their way to a hideout where they suspect Taeyeon is being held...