Hokkaido, Sapporo. High school student Ruka is living her life in bliss in the house of lilac. Even though they didn't share their mother, Ruka had a beautiful older sister, as well as a lovely younger sister. They lived with their strict but loving mother and their gentle father, whom they all loved dearly. One day a young boy named Shion transfers to Ruka's school. For some unknown reason, he views Ruka as his sworn enemy. However, along with solving the riddle of his behavior, Ruka's life begins to turn for the worst...