Rika visits her best friend Reiko for dinner and is captivated by a home-cooked meal for the first time in years.

Reiko advises Rika to write to Kajii asking for a recipe, offering her a way into the case.

Rika meets Manako Kajii at the Tokyo Detention House, and Kajii tasks her with making a simple buttered rice dish.

Rika uses her kitchen for the first time in years and cooks Kajii's buttered rice, overwhelmed by the flavors.

Kajii instructs Rika to eat butter ramen at a late-night Shinjuku restaurant only after having sex.

Rika rents a hotel room, sleeps with Makoto, then sneaks out alone while he sleeps to eat the ramen.

Rika and Shinoi bake a quatre quarts cake together in his spare apartment near the office.

Rika heads to Niigata on assignment and is surprised to find that Reiko has joined her on the train.

Rika and Reiko visit Kajii's family home in Niigata and meet Kajii's sister Anna and their mother Masako.

Rika visits Kajii and discovers that Reiko has been secretly visiting the Detention House without telling her.

Rika and Kitamura travel to Shirō Yokota's house and retrieve Reiko, who has gone completely silent.

Rika meets Makoto in person to talk and they officially break up, with Rika briefly hoping he will profess his love.

Rika and Reiko begin attending cooking classes at Le Salon de Miyuko under false names.

Rika visits her father's grave for the first time in eight years and reflects on their troubled relationship.

Rika hosts a turkey dinner for her friends and family in her newly purchased three-bedroom apartment.

Rika Machida, a Tokyo journalist, is drawn into the case of Manako Kajii, a food blogger convicted of killing three men, and secures a meeting at the Detention House. Under Kajii's guidance, Rika begins cooking and eating new foods, transforming her restricted and isolated lifestyle. A trip to Niigata with her best friend Reiko uncovers disturbing truths about Kajii's past while also deepening Rika's personal reckoning with herself. After retrieving Reiko from a dangerous solo investigation, Rika ends her unfulfilling relationship with Makoto and begins building a new community with her friends. She infiltrates Kajii's former cooking school and grows to understand the truth behind Kajii's loneliness and desires. The novel concludes with Rika hosting a lavish turkey dinner in her new apartment, celebrating a hard-won sense of identity and her capacity to love and care for others.

The headline events of Rika's investigation and transformation.

Rika searches the market for high-quality butter and finds only margarine due to city-wide shortages.

After Kajii refuses her visits for weeks, Rika sends food treats to Kajii through the Detention House shop.

Rika makes tarako pasta using Kajii's blog recipe and reflects on the stories surrounding Kajii's alleged victims.

Kajii writes to Rika asking her to find and eat a specific Christmas cake from West Bakery and report back.

Ryōsuke, who works for a confectionery company, invites Rika to his facility so she can taste the sold-out West Bakery cake.

Rika visits her mother Misaki and makes toasted buttered mochi to cheer her up after a difficult day.

Rika and Makoto go out for dinner, where he delivers a lengthy lecture about women's image and self-control.

Rika meets her former colleague Mizushima for a meal and he reassures her that she looks healthy, not careless.

Rika goes to Makoto's office and gives him the Valentine's Day cake, insisting she is not trying to serve him.

Reiko breaks down at the Niigata hotel, admitting she joined the trip to escape her difficult marriage to Ryōsuke.

Ryōsuke arrives at Rika's office to report that Reiko has not come home and cannot be contacted.

Reiko finally breaks her long silence at Shinoi's apartment, speaking openly about her marriage and her hopes for Rika.

Rika and Chizu go for coffee after cooking class and Chizu explains exactly how Kajii alienated every student in the school.

Rika's editor, pleased with reader response to the serial piece, asks her to expand the Kajii article to six installments.

Rika prepares asparagus with hollandaise sauce and shares it with Shinoi, and the two feel closer as a result.

Behind the main events of the investigation, a series of smaller but important scenes trace the texture of Rika's transformation. Early forays into markets, a sold-out Christmas cake, and tentative experiments with pasta mark the hesitant beginnings of her culinary awakening. Key interactions with her boyfriend Makoto, her mother Misaki, and her reassuring colleague Mizushima reveal the social pressures Rika navigates as she begins to change her habits and appearance. The Niigata trip surfaces the depth of Reiko's marital unhappiness, and Reiko's eventual return to herself brings the two friends closer than they have been in years. As Rika's serialized article about Kajii gains momentum and she forms new alliances with colleagues like Chizu and Kitamura, cooking and community quietly become the foundations of her life.

The quieter but significant moments shaping Rika's culinary and emotional journey.

During their first meeting, Kajii passionately rants about the difference between real butter and margarine and declares her hatred of feminists.

Makoto visits Rika's apartment, eats the pasta she cooked, then coldly tells her she doesn't need to be domestic.

Rika meets Shinoi for drinks and they discuss Kajii's case, public opinion, and the human fear of dying alone.

Kajii tasks Rika with dining at the upscale French restaurant Robuchon, where Kajii often went with her last alleged victim.

Rika dines alone at Robuchon and is so overwhelmed by its richness and opulence that she finds herself wanting more.

Reiko confesses she has been attending fertility clinic appointments alone, revealing the true and troubled state of her marriage.

While visiting a dairy farm near Kajii's hometown, Rika is suddenly struck by a flashback to finding her father dead in his apartment.

Anna reveals that a sex offender attacked her when she was young and that Kajii devoted herself to caring for the man afterward.

Reiko secretly travels to Shirō Yokota's house and moves in for three days, convinced he helped Kajii murder her victims.

Reiko realizes that her experience cooking and cleaning for Yokota is indistinguishable from her marriage to Ryōsuke.

Kajii hints to Rika that there is a fourth tiger in the story, alerting her that someone else is involved in the case.

Rika realizes she does not love Makoto during his frantic phone call and resolves to end the relationship in person.

Rika confronts colleague Kitamura about spying on her and enlists his help to locate Shirō Yokota.

Kajii refuses to see Rika at the Detention House and Rika, standing immobilized outside, is struck by a passing car.

After the turkey dinner ends and her guests sleep, Rika lies awake inventing her own new recipe using the leftover turkey.

Butter is propelled by a sequence of revelations that shift Rika's understanding of Kajii, herself, and the people she loves. From Kajii's provocative declarations about butter and feminism to Reiko's confession about her secret fertility appointments, each discovery forces Rika to question her deepest assumptions. A solo dinner at Robuchon and a late-night ramen experience become turning points in which Rika begins to claim pleasure and freedom on her own terms. The trip to Niigata and Anna's shocking account of Kajii's formative past expose the roots of Kajii's destructive worldview, while Reiko's solo investigation leads to her own painful revelation about servitude and self-worth. Rika's discovery that a fourth person is entangled in Kajii's story, followed by a sudden moment of crisis outside the Detention House, push her toward acceptance and decisive change. By the novel's final scene, Rika's quiet act of inventing her own recipe in the dark confirms her complete liberation from others' expectations.