The story introduces Satsuki Aizawa, a diligent stationery designer who, by night, is the popular doujin artist "Kuroi-san." Satsuki meticulously leaves work at 5 PM, a condition for her employment, which also allows her to pursue her side job. She is exhausted by her family's constant pressure to marry, especially during her annual summer visit to their rural inn, and openly wishes for a "fake marriage" to silence them. Her doujin assistant, Warabi-chan, is a long-time friend who knows her only by her pen name.
At a doujin event, Satsuki's wish takes an unexpected turn when Ryuta Takimoto, a cool, quiet, and high-achieving salesperson from her company, approaches her in full otaku gear (cameraman for cosplayers) and proposes marriage. Takimoto reveals his secret identity as a Vocaloid producer ("Takapi") and passionate idol otaku. His motivation for a quick marriage is to reassure his mother, who has put her own long-term relationship on hold for his sake. Satsuki, initially bewildered, is intrigued by his pragmatism and shared "otaku" sensibilities.
Over tea, Takimoto reveals he recognized Satsuki from a Comiket event last winter, where she was sleeping with a prototype iPad cover (which he invented and everyone else had mocked) on her face. Satsuki, who had been the only one to praise the cover, is surprised. Recognizing their mutual need for a convenient arrangement, Satsuki proposes they try living together first in her large, isolated, two-family house on a steep hill, suitable for a "share house marriage." Takimoto, who has secretly admired Satsuki since her praise of his iPad cover, readily agrees, thrilled by the unexpected opportunity.
Takimoto moves in, surprising Satsuki by arriving on a road bike. He immediately notices and appreciates the stunning view from her house, a detail Satsuki loves but has never had anyone else acknowledge due to the house's difficult access. This shared moment deepens Satsuki's belief that Takimoto might be a suitable partner. They bond further while cutting bamboo for a rain gutter (revealing Satsuki's rural inn upbringing and surprising skill with tools) and attempting nagashi somen, discovering a mutual love for crafting. Takimoto shares details of his complex idol fandom, and Satsuki, despite her different genre, finds herself engaged. They establish pragmatic rules for their cohabitation, prioritizing individual hobbies, open communication, and flexible schedules, which brings Takimoto unexpected comfort, having grown up in a quiet, lonely home.
Satsuki agrees to meet Takimoto's kind and gentle mother, a stark contrast to her own critical family. During the meeting, Satsuki discovers that Erika, the daughter of Takimoto's mother's partner, is the same high school girl she helped at a copy shop earlier that day. Erika, also a secret doujin artist, accidentally left her USB (containing BL doujinshi data) in a gift bag for Satsuki. Understanding Erika's predicament, Satsuki, with Takimoto's help, retrieves and returns the "life-saving USB," offering empathetic advice on hiding her hobby from family. This act solidifies a bond between Satsuki, Takimoto, and Erika, highlighting their shared understanding of otaku life.
The initial "fake marriage" premise evolves into a genuine, if unconventional, partnership built on mutual respect, shared interests, and a deep understanding of each other's unique needs and passions. Takimoto's secret love for Satsuki deepens as he experiences the joy and comfort of her presence, while Satsuki, initially cynical, finds unexpected connection and support in their shared life.
