The top student in Class S of Jeonghan High School mysteriously dies. Knowing the real cause of death, Min-young struggles to cover up the incident.

Soo-young happens to give the mysterious cookie to someone at work. But this one cookie becomes a threat to the sisters' lives.

While investigating Yoo-jung's death, a detective notices that the cookie is at the center of the case. Min-young attempts to derail the investigation.

A detective visits Soo-young after discovering the cookie's connection to Min-young. Infuriated by the accusation, Soo-young looks for the truth.

Seong-pil, a consultant at the school, calls Min-young privately and offers to help her. Meanwhile, Soo-young's investigation of the cookie leads her to the school.

With Seong-pil's help, Min-young tries to get out of the cookie business and turn herself in. However, her plans go awry when she encounters an unexpected person.

Soo-young heads to the school to avenge Min-young and takes Won-ju, a cookie distributor, hostage. The chef doesn't budge, and Ho-soo gets caught up in the fight.

To save Min-young, Soo-young decides to become an employee of High Cookie. She disguises herself as a transfer student and sells the cookies at Jeonghan High School.

Soo-young's seemingly smooth business faces a crisis as she gets involved with Jin-woo, the chairman's grandson. Is it all a part of Soo-young's operation?

Through a sweet school life with the cookies, Soo-young often forgets about Min-young. Meanwhile, Seong-pil tries to use Soo-young to find Chef.

Soo-young is confused when she discovers Chef's true identity. In hopes that it's not true, she tries to test him, but Chef falls into Soo-young's trap.

Soo-young learns that Chef can't create an antidote to save Min-young. Despite a momentary setback, Soo-young tries to obtain the antidote in her own way.

Soo-young needs the antidote, and Seong-pil needs the recipe. The two are now in the race to get to Ho-soo first.

Soo-young and Ho-soo hide from Seong-pil. The two start investigating Seong-pil to find out his weakness.

Min-young wakes up in an unknown place. Meanwhile, Soo-young negotiates with Seong-pil to find out where Min-young is.

Soo-young goes to the place where Min-young is supposed to be, but she is not there. Meanwhile, Ho-soo and Seong-pil fight for their lives.

Soo-young returns to her ordinary life with Min-young. Before they know it, the sisters' positions are reversed. Soo-young starts to miss the cookie's taste.

Soo-young realizes that she can never go back to her nice life without the recipe. But if she gets the recipe, does that mean she can live the life that she wants?

People are now looking for the cookie recipe. Chef asks Soo-young if she's confident enough to handle the recipe.