What happen when you fall in love with someone that you shouldn't fall for?

What happen when you fall in love with your sister's fiance?

Should you fight for the one you love or sacrifice your happiness for your beloved?

CHARACTER:

JUNG SOOJUNG aka KRYSTAL

• fashion designer who just come back from Paris
• fall in love with her sister's fiance
• has best friend, Oh Sehun
• beautiful, elegant
• loves her stepsister, Choi Sulli so much
• willing to sacrifice everything for her beloved

KIM JONGIN aka KAI

• famous photographer
• fall in love with his fiance's sister, Jung Soojung
• Choi Sulli's fiance
• think Sulli as his sister only
• hates his father very much
• handsome
• has best friend, Oh Sehun, lives with him

CHOI SULLI aka SULLI

• doctor
• pretty
• loves Kim Jongin and Jung Soojung so much
• very spoiled 

OH SEHUN aka SEHUN

• CEO in his father's company
• handsome
• loves Jung Soojung
• Kim Jongin's best friend and live with him