The events in Fushidara na jo Nico ("Fallen Woman Nico") take place in Otonokizaka, an old and traditional girl's school in Akihabara, Tokyo. One of the second-year students, Honoka Kousaka, recruits eight other students to form a singing and dancing "school idol" group. After two girls named Maki Nishikino and Nico Yazawa are recruited, they become a couple, but Maki eventually finds Nico to be clinging and suffocating, and takes a break from their relationship. Nico believes Maki has abandoned her and breaks down. The idol group has become a kind of family by that point, so the other seven girls become concerned about their break-up, and especially about Nico.
In The Book of NicoTori, Kotori Minami, the daughter of the school principal and a member of their idol troupe, is discussing the situation with two long-time friends, the group leader, Honoka, and Umi Sonoda, a prince-like girl very popular at their school. Honoka somewhat jokingly suggests Nico should date the other girls in the group (which they named "Mu's," a pun on "Muses") until she feels better. To Umi's discomfort, Kotori says it's a great idea, and asks Nico out.
It turns out that Kotori has a plan that she recruits Honoka and, grudgingly, Umi into. She proposes that they be part of a couple with Nico, sequentially, and only for a week each. In order to experience what that's like, and demystify it somewhat, she treats Nico as a full girlfriend and lover for the entire week, and recruits as many students as she can to help Nico restore her confidence and re-establish her life.
Meanwhile, Nico's now ex-girlfriend Maki feels alienated from the other students and cast in the role of being cold-hearted to Nico. She retreats into herself almost as badly as Nico did. Eli, one of the older members of the group, and Umi both communicate with Maki trying to reassure her and cheer her up. Nonetheless, to Maki's displeasure, Kotori, Honoka and Umi agree to go along with Kotori's plans and that Umi will be her next girlfriend. The still heavily depressed Nico goes along passively with anything anyone suggests to her, but is clearly getting better, and she attributes that in part to the "Nico dating project."
The Book of UmiNico starts after Nico and Kotori have finished their one-week relationship. It is the morning of the next day, and Kotori is handing Nico over to Umi. Umi has introduced a wrinkle to the plot, however: she feels Nico has avoided her issues with Maki long enough, and it is time they faced each other, so she invites Maki to meet her, Nico and Kotori for breakfast. Honouring a request Nico's mother made of Kotori, Umi also urges Nico to consider carefully if the project will really help her more than it might harm her.