Yoon Se Na (Krystal Jung) thinks she can finally live happily ever after with Lee Hyun Wook (Rain), the man she loves with all her heart. But, the unexpected happens. 
Yoon Se Na finds out she has another older sister, and not the dead sister that was Hyun Wook's ex-girlfriend. Her mother had another man before she and her older, but dead sister was born.
Kang Jong Hee (Jessica Jung) is Yoon Se Na and dead Yoon So Eun's older sister. After her parents split up, she went off to live with her dad. She discovered love, but was soon heartbroken. Later, she lets go of him. Jong Hee has a bipolar disorder, and so after some therapy, she's finally getting better. 
When they meet, sparks fly, hatred is burned into their hearts, forbidden feelings start to grow, hearts get broken, and so much more.