The Reason My Boyfriend Married My Older Sister

Everything was going well in both her love life and her career. That was until that day came. Miyata Aiko (Akita Shiori), who works at an apparel store, lost both her parents at a young age, but lived a happy life with her kind older sister Saeko (Takada Riho) and her childhood friend and boyfriend Yoshimura Yuma (Kageyama Takuya), a doctor. Then, just before her 25th birthday, Aiko is told by Yuma, "I have something important to tell you," and her heart starts beating fast, thinking, "Maybe he's going to propose to me?" However, one day, on her way home, someone pushes her from behind, causing her to fall down the stairs and fall unconscious! When she wakes up, she learns an unbelievable truth...

Aiko (Akita Shiori) is called to a hotel room by her ex-boyfriend, Yuma (Kageyama Takuya), who is now her sister Saeko's (Takada Riho) husband. For some reason, she meets Yuma's father, Masaki (Maruyama Tomoki) there, but suddenly loses consciousness. When she wakes up in bed in the morning, she finds Masaki sleeping half-naked next to her...! Aiko can't accept this reality, but later, while she is working at the apparel store where she works, Yuma's mother, Yuriko (Akama Mariko), appears and confronts her with a photo...! Furthermore, Aiko goes to see Masaki, who claims to know the person who pushed her down the stairs. But who is that culprit...? And what exactly are Yuma's true feelings...?!

It's been a month since Yuma (Kageyama Takuya) and Saeko (Takada Riho) suddenly disappeared. Now alone, Aiko (Akita Shiori) has been transferred to a new department. Meanwhile, a new part-timer, Makino Takatoshi (Ikeda Masashi), arrives at her workplace. Aiko feels as if someone is following her on her way home from work at night, so she asks Takatoshi to escort her. Aiko's heart slowly begins to open up to the kind and cheerful Takatoshi. Aiko tries to forget about Yuma and move on, but...

Aiko (Akita Shiori) has started dating Takatoshi (Ikeda Masashi). She tries to forget about Yuma (Kageyama Takuya) and focus on Takatoshi. But one day, when Aiko gets out of the bath, she finds over 100 missed calls from Takatoshi on her smartphone...! After that, Takatoshi's love for her becomes too intense, and his abnormal behavior and controlling behavior towards Aiko escalates! Then, in the midst of a certain incident, Aiko witnesses Saeko (Takada Riho) in an unbelievable state...

Aiko (Akita Shiori) , who witnessed the shocking state of her sister Saeko (Takada Riho) , is shocked to learn from Yuma (Kageyama Takuya) that Saeko and Takatoshi (Ikeda Masashi) were secretly connected and the truth behind Aiko's fall... Aiko cannot accept that her beloved sister Saeko, who she is close with, tried to kill her. Meanwhile, Saeko, who has lost Yuma, goes to see him after work and takes an outrageous action!? Meanwhile, Takatoshi arrives at Aiko's place as she is waiting for Yuma!

While Yuma (Kageyama Takuya) is being held captive by Saeko (Takada Riho), Aiko (Akita Shiori) is visited by Yuma's mother, Yuriko (Akama Mariko). Yuriko learns that Aiko was not her husband Masaki's (Maruyama Tomomi) mistress and apologizes to Aiko. Yuriko also has evidence of Saeko and Masaki's relationship, and Aiko decides to confront Saeko once again... Aiko makes up her mind and goes to see Saeko, but is shocked to hear about Saeko's past... Meanwhile, Saeko declares to Aiko, "Miyata Aiko will die soon"...!?

Aiko (Akita Shiori), who was held captive under Saeko's (Takada Riho) control, had her sense of self-worth denied, her dignity as a human being taken away, and she lived a dark life. However, one day, when she hears the voice of Yuma (Kageyama Takuya), who has come looking for Aiko while Saeko is away, Aiko makes a decision. Aiko appears before Yuma and begins to tell him everything that has happened up to that point, but it is an extremely shocking truth...! Not wanting to involve him any further, Aiko kicks Yuma out and stays in the house where she was held captive, but it seems she has a certain idea...!?

Saeko (Takada Riho) chokes Aiko (Akita Shiori) and says, "You lost," with a look of hatred on her face. However, Aiko still tells her that she loves her sister, even after learning everything. As Aiko loses consciousness and nears the end of her life, what will happen to her? Yuma (Kageyama Takuya) rushes to her side and apologizes for not noticing Saeko's suffering, but Saeko leaves the scene. Immediately after, something strange happens outside...! What will be the outcome of Saeko's sad revenge...? And how will the relationship between Aiko, Saeko, and Yuma end?