On her eighteenth birthday, Ami Nonomura decides to travel in search of her estranged father and brother. During her journey, she is attacked by a thief but is rescued by a young man named Hiroshi Tazaki. A romantic connection quickly forms between them, but their happiness is shattered when they discover they are actually siblings.

Raised in China, Ami is a high school student skilled in Chinese martial arts and swimming. She returns to Japan following her mother's remarriage. Hiroshi is a careless college student with a habit of secretly photographing women. After Hiroshi's father and Ami's mother marry and depart on their honeymoon, the two new step-siblings are left alone together in the family home. Whenever Hiroshi attempts to get close to Ami, her martial arts training keeps him at a distance, though they gradually begin to grow closer.

The narrative then shifts forward one year. Ami is now living with Hiroshi and his wife, Mariko. An undeniable tension simmers between the two women, which eventually boils over when Mariko becomes angry with Ami. The story revolves around the complex and taboo relationship between the step-siblings, exploring themes of forbidden love and family secrets.