※This synopsis, intended for public submission, summarizes the story from beginning to end, and therefore contains spoilers.

Kaito Tsurugi has two childhood friends: Allan Kisaragi and his older sister, Ellen Kisaragi. Every girl Kaito takes an interest in ends up falling for Allan.

Ellen, who comforts Kaito, is secretly the mastermind, though he remains oblivious. Ellen encouraged Kaito, who was wounded by the betrayals of the 'gal' Minase Yuki and his homeroom teacher Himemiya Chika, but it was all merely part of her plan.

Ellen's transformation into a yandere is linked to her past. In elementary school, Ellen, who was bullied, was helped by Allan and Kaito, but one day she faced Allan's betrayal.

Hating Allan, Ellen was saved by and fell for Kaito, who remained her unwavering ally. However, in her third year of middle school, she completely shattered when Kaito got a girlfriend. Afterward, Ellen, who held Allan's weakness against him, forced him to steal Kaito's girlfriend.

As Kaito's wounds began to heal, he was courted by transfer student Hikaru Hiryu and they started dating. Allan's temptations proved ineffective against Hikaru, but she was eliminated by being ensnared in a trap.

Kaito, who had to recuperate from the shock, was nursed by Ellen and his cousin/first love, Yukina Tsurugi, and gradually regained his health.

However, he was bewildered to realize that unsettlingly dark emotions were emerging towards Ellen. Amidst this, Kaito was confessed to by Yukina, but far from feeling pleased, he felt a sense of discomfort, ultimately leading him to reject her.

When Kaito felt nothing upon witnessing Yukina and Allan kissing, and found himself bewildered, Ellen pointed out that he was "broken," and he finally became aware of his own abnormality.

Kaito finally started dating Ellen, but Allan tried to tear them apart. After numerous twists and turns, Allan succeeded in exposing Ellen's true nature before Kaito, but it proved meaningless to the now "broken" Kaito.

An enraged Allan attempted to kill Ellen but failed when Kaito protected her. Thereafter, Kaito and Ellen, united in both mind and body, began cohabiting upon high school graduation and married upon university graduation. The story concludes with the two, blessed with a child, watching over its growth.