The story picks up with the protagonist, Haru, in the literature club with his junior, Minase Mitsuki. It's Haru's birthday, and Mitsuki brings him coffee as a gift, despite Haru's known aversion to caffeine due to a physical reaction that causes headaches. However, upon drinking the coffee, Haru experiences a pleasant, lightheaded sensation, leading him to believe his tolerance might have changed. Mitsuki, noticing his reaction, playfully suggests she could be his birthday present, causing Haru to blush. He drinks more coffee, and his memory of the rest of the day is lost.
Haru wakes up the next morning to find Mitsuki in his apartment, having made him breakfast. She acts familiarly, calling him by his first name and revealing she has a key to his place, stating they are getting married. Haru is bewildered, as he has no memory of agreeing to any of this, attributing his lost memories to the coffee. Mitsuki, however, insists they made a strong promise to date with marriage in mind. She then leaves abruptly for an early appointment, promising to walk to school with him tomorrow.
Later, Haru is met at his door by his childhood friend, Mei Kisaragi. She also claims they are going to school together and reminds him of a promise they made. Haru, still reeling from Mitsuki's claims and his memory loss, is further confused when Mei reveals their promise was to get married at 18 and date until then. He realizes his coffee-induced amnesia has led to him agreeing to relationships with both Mitsuki and Mei.
At school, Haru's friend Kyosuke teases him about walking to school with Mei, assuming they are dating. Haru tries to deny it, but Kyosuke is unconvinced. During lunch, Mitsuki invites Haru to eat the bento she prepared, reiterating that she is his "girlfriend." Haru feels increasingly overwhelmed by the situation and the implications of his lost memories.
Later, Mei confronts Haru about missing their lunch plans, revealing she was upset. She also brings up Luna Lilian, a former classmate who has returned to Japan. Mei expresses jealousy and dislike towards Luna, whom she perceives as manipulative. Haru, meanwhile, encounters Luna, who claims he helped her the previous day and that they are destined to be together, showing him his student ID which he apparently dropped. Luna also reveals a handmade marriage certificate from their childhood, adding another layer to Haru's predicament.
Haru attempts to explain his memory loss to both Mei and Luna, but their reactions are varied. Mei, surprisingly, accepts his explanation about the coffee, attributing it to his known sensitivity. She then declares that their relationship will officially begin that day, as his confession was made while he had no memory. Luna, on the other hand, is heartbroken by his lack of recollection.
The following morning, Haru wakes up to find Mitsuki has made him breakfast again and insists on walking to school with him, reinforcing their "relationship." He also receives numerous messages from Luna. Haru tries to manage the escalating romantic entanglements, sending a quick message to Mei to go ahead to school without him. He then replies to Luna, explaining his condition.
At school, Kyosuke continues to tease Haru about his relationships. Haru learns that Luna has been asking around the school for him. During lunch, Mei takes Haru to the rooftop for a private meal, expressing her long-held feelings for him and their impending marriage. Haru, overwhelmed, struggles to process the events. He also learns that Mei practiced cooking his favorite meal, hamburgers, for him.
Haru finally confesses his memory loss to Mei, who, after a moment of silence, accepts his explanation, stating that his coffee sensitivity is a known issue. She then declares that their relationship officially starts that day, as his previous confession was invalid due to his amnesia. Mei reveals she has waited five years for him and that they are going to get married. Haru feels like he is being swept into a story he cannot control. The chapters conclude with Haru grappling with the realization that his life has become significantly more complicated due to his coffee-induced amnesia, leading to multiple romantic entanglements.
