Betrothed to my Sister’s Ex

Marie, the second daughter of a poor baron family, was forced to wear tattered clothes and was treated like a servant by her own parents. One day, Marie meets the incredibly wealthy Count Kyuros at a birthday party. Though they are drawn to each other, Kyuros, due to a misunderstanding, ends up proposing to Marie's older sister, Anastasia!

Rejected by Marie, Kyuros is confronted with a harsh reality by his head chamberlain, Mio, and hangs his head in dejection. Hating the constraints of nobility, he has always avoided marriages of pure political convenience. Now, his past is revealed: why he has lived this way, how meeting Marie changed him, and the true extent of his feelings for her.

Marie becomes Kyuros's official fiancée and begins her new life at Granado Castle. Though bewildered by the luxurious meals and clothing she's given, she tries her best not to be a burden to the servants. However, her new attendants have a somewhat sarcastic attitude. On a sleepless night, she runs into Kyuros in the dining hall, and they end up talking about each other's "favorite things"...

Kyuros wants to offer Marie the finest hospitality, while Mio sees her own background reflected in the timid Marie. The two of them resolve to create a comfortable environment where Marie can feel at ease. But the next day, a package and a letter filled with malice arrive for Marie from the Baron's household.

Marie and Mio are invited to a swordsmanship exhibition by Kyuros's old friend, Louis-phon. Although Marie is frightened by the dangerous competition, she finds herself moved by the gentlemanly conduct of the Dark Warrior. After the match, Marie reunites with Kyuros, only to be suddenly and harshly condemned by Louis-phon for their ""loveless political marriage"".

Marie's heart begins to flutter whenever she thinks of Kyuros. In the library, she bumps into Kyros's mother, Ryuu-Ryuu. Ryuu-Ryuu is surprised by Marie's extensive knowledge, and while suspicious of why she was involved in managing the baron territory, she speaks to Marie with kind words that help open her heart. Meanwhile, on Kyuros's orders, Mio is also investigating the baron's family...

A letter from Marie's brother, Cedric, states that Anastasia, who was supposed to be dead, has returned to the Shaderan family. Finding the contents hard to believe, Mio hides her identity and visits Marie's family to ascertain the truth. What is the Shaderan family hiding!? Meanwhile, at Kyuros's invitation, Marie heads to the royal capital. A date for just the two of them begins.

To meet with Louis-phon, Marie and Kyuros visit a jeweler that serves the royal family. While dumbfounded by Louis-phon's surprise, Marie strengthens her resolve to live as a member of the Granado family, and her feelings for Kyuros deepen. However, just as she's finally able to savor her own happiness, her feelings of guilt towards Anastasia surge back...

The man brought before Kyros was Jacob, the coachman of the carriage Anastasia was riding in on the day of the "accident". With Jacob refusing to speak, Johan, who is from the same hometown, puts on an act to draw out the truth. The full account of the accident that Jacob tells is beyond Marie's imagination, and Kyuros's own perception of Anastasia is also greatly shaken.

Anastasia is alive? However, the apprentice of Norman whom Marie suspected turned out to be a boy named Arthur. Still unconvinced even after hearing Louis-phon's story, through the arrangements of a worried Kyuros, Marie decides to meet with Norman in person... Although Norman insists that his disciple is a boy, the description he gives only deepens Marie's suspicions.

The one who attacked Marie was Anastasia! And she demands that Marie and Kyuros annul their engagement. For the time being, Marie guides Anastasia—who, for some reason, looks at Kyuros with hatred—into the castle. Then, Anastasia begins to speak of a past in which she "failed to protect" Marie. What is the reason Marie's life was turned upside down, leaving her so "tattered"...!?

Marie decides to settle things with the Shaderan family once and for all. With the help of Kyuros and everyone, Marie forces her way into the mansion where her father, Gregor, and the others have barricaded themselves. Gregor continues to resist even when confronted with his crimes, while her mother Elvia's secret is revealed in front of Anastasia. How will Marie confront parents like these?