Fukazawa Yumi's tie is fixed by Ogasawara Sachiko, her idol and the school's star, setting off a chain of events which would throw the shy and ordinary girl into the world of the prestigious student council. After Yumi walks in on Sachiko arguing with the Yamayuri Council over the school play they will be performing, Sachiko suddenly asks Yumi to be her petite soeur in an attempt to force her way out of the lead role.

Yumi is hounded by the school newspaper for a report on why she turned down Sachiko and endures dance lessons with the Yamayuri Council and a piano duet with Sachiko. Through various people, Yumi learns more about Sachiko and Suguru Kashiwagi, the one who will be playing the Prince in the Cinderella play.

Yumi finally meets Kashiwagi when he comes to Lillian for the dress rehearsal, and learns a shocking secret about his relationship with Sachiko. Sachiko's true reason for her refusal to play the lead role is understood, making the bond between her and Yumi truly genuine.

The rest of the Yamayuri Council find out that Yumi finally accepted Sachiko's rosary. Meanwhile, Shimazu Yoshino and Hasekura Rei are voted the school's best sœurs due to their respective girly and bishonen appearances, but Yumi talks to Yoshino and finds out that there's more to the pair than meets the eye and that the relationship between the two is far from perfect.

While Rei is reeling over Yoshino's shocking announcement, the bulk of the school copies them by breaking their soeur relationships too. Tori Eriko, Rei's soeur, isn't even aware of the problem due to her odd behavior and suspiscious absence from school. Yumi goes to the hospital to talk with Yoshino, and learns the true nature of the Yellow Rose soeurs. Yoshino tells her all about her plan to make Rei a stronger person.

A second-year student calling herself Rosa Canina runs against the en boutons in the Yamayuri Council elections with murky intentions. Todo Shimako hesistates to run at all due to her first-year staus and her reluctance over taking a leading council position until a talk with Rosa Canina forces her to make a decision. Yumi confronts Sato Sei, Shimako's soeur, over her refusal to encourage Shimako to run, and learns more about the relationships between older and younger soeurs and how to best support Sachiko.

For Valentine's Day, the Lillian Newspaper Club organizes a treasure hunt in which the winners would get half-day dates with the en boutons. Yumi agonizes over what to get Sachiko for Valentine's Day, resulting in a spectacular misunderstanding between the two of them.

After a crying Yumi runs away from Sachiko's confrontation, Sei reveals the secret reason behind Sachiko's distress. The treasure hunt takes place with surprising results. and Yumi makes up with Sachiko in her own unique way.

Uzawa Mifuyu narrates her past as Sachiko's desperate admirer and her reasoning behind her decision to not claim the treasure hunt's prize. She finally acknowledges that Yumi is a better match for Sachiko than her, and quietly slips out of their lives and the series.

A tragic lesbian romance novel causes a scandal at Lillian because the author is rumored to be none other than Sei. This prompts Sei to tell Yumi and Yoshino about her past with a girl named Kubo Shiori.

A flashback from a year ago tells the story of an angry, silently-suffering Sei, who is saved from her alienation by a girl called Kubo Shiori. The two of them begin a passionate, all-consuming romance, but Shiori's secret plans for the future shatter Sei's heart.

The day of the Boutons' dates arrive, and each pair has their problems. Yumi worries over where to take someone of Sachiko's stature, Yoshino follows Rei's date around in her possessive jealousy, and Shimako braces herself for spending a day with her rival, Rosa Canina. Meanwhile, Tsukiyama Minako, the head of the Newspaper Club, and Takeshima Tsutako, the school's ace photographer, try their best to get the scoop on the dates, but they comically fail to track any of them.

Tsutako and Minako's papparazzi-like plans wear them out while Shimako comes to an understanding with both Sei and Rosa Canina. The dates ultimately strengthen Yumi and Yoshino's relationship with their soeurs and reaffirm the place they stand in their older sister's lives.

Sei takes Yumi to visit Sachiko's house on New Year's Day where the Yamayuri Council have a party. All the major characters are reintroduced, and with the start of the New Year, Yumi looks happily forward to spring.

A series of photographs implicate Eriko in enjo kosai, or compensated dating, while Eriko's behavior becomes even more erratic and strange than usual. As it turns out, Eriko does have a secret, but its far from what she has been accused of.

In their midst of their graduation preparations for the third-years, the en boutons forget to organize a farewell party for the Rosas. To make it worst, Yumi finds out at the last moment that the en bouton petite soeurs must also put on a performance, and collapses from the pressure of all her various duties.

In the days before graduation, Rosa Chinesis and Rosa Foetida leave their parting words to their petite soeurs and grande-soeurs. Yumi begins to worry why Rosa Gigantea hasn't done the same. She swears to Sei she'd look after Shimako and promises to do anything as her final request, and Sei's answer leads to a surprisingly sweet moment and a revelation about her future.

Graduation day arrives. As the rest of the Yamayuri Council struggle with conflicting emotions, the Rosas reflect on their lives and the memories which shaped them.

With graduation's onset, Sei and Shimako remember the events which led them to become soeurs in spite of their many complex obstacles and reservations. The two of them ultimately form a deep and healthy relationship without becoming codependent.

Shimako falls into a depression after Sei graduates and cannot bear the weight of her secret alone. But a chance meeting with the down-to-earth Nijo Noriko gives her a new friend.

Shimako and Noriko become even closer, but a shocking revelation about Shimako's true religion is announced in front of the entire school, testing their relationship. Meanwhile, the Yamayuri Council is concerned over the state of Shimako's well-being and decide to take matters into their own hands.

Shimako is pressured to take Noriko as her petite soeur, but is hesitant due to the differences between Noriko and the general culture of Lillian. Caught between her loyalty to the Yamayuri Council and her desire to protect Noriko, Shimako quietly suffers until an argument forces Noriko into action.

Yoshino takes advantage of her newfound health by joining Rei's kendo club, causing extreme worry on Rei's part and another roadblock in their relationship.

Yumi and Sachiko have not been spending much time with each other, for Sachiko is always with Tōko. Every time Sachiko and Yumi are alone together, Tōko appears. Yumi begins to believe that Sachiko prefers Tōko over her.

Yumi gets back onto her feet with different people helping her out. She learns that she must develop an identity outside of being Sachiko's petite soeur.

Yumi learns why Sachiko has been so distant lately. With newfound confidence, she promises never to question Sachiko's love for her again.