Shiki Tohno suffered a severe injury during his childhood, and as a result, he was sent away from the Tohno family home to live with a relative. Years later, while Shiki is attending high school, his father—the head of the Tohno household—passes away, and Shiki is ordered to return by his sister Akiha, who has taken over as the new head of the family. But Shiki carries a heavy secret. Since that injury, he has been able to perceive lines on everything around him, and only a special pair of glasses allows him to suppress this ability. He also has very little memory of the time before the accident. On the day he moves back to the Tohno estate, he encounters a woman named Arcueid Brunstud and, in a momentary loss of control, stabs her through the neck with his knife, seemingly killing her. However, when she later appears beside him unharmed and asks him to become her bodyguard, Shiki's quest to uncover the truth about his past begins.