Krystal (You) is a tomboy whose childhood friend is the handsome Kim Myungsoo or known best as , L. This friendship is tested when L admits that he has fallen in love with Sulli , a mysterious comic writer but very beautiful & feminine. Wrecked by jealousy , Krystal has an accident & is forced to have her legs amputated. As fate would have it , Sulli is also seeking treatment for a heart disease at the same hospital as Krystal. A difficult choice lies ahead of Krystal. Would she be willing to make the biggest sacrifice & donate her heart to Sulli , just to see L , her bestfriend , find his happiness?