Riiko Izawa has never had a boyfriend and she has been rejected by every boy she has ever had a crush on.
When she returns a lost cell phone to an oddly dressed salesman, she mentions wanting a boyfriend. To thank her, he directs her to his company's website, Kronos Heaven. When she visits the site, Riiko finds it's a site to create your perfect lover. Thinking it's a game, she customizes and accidentally orders one. The next day, her new lover arrives. Following the instruction manual, she kisses him to wake him up and configures him to be in love with only her. She names him "Night." Three days later, she is shocked to learn that she only had him for a free trial for 72 hours. The salesman, Gaku Namikiri, tells her that she must now pay 100,000,000 yen for Night, however, they will waive the fee if she helps them to collect data about how women think and feel to help perfect future models.
Riiko must now keep Night's true nature a secret from everyone around her. She also soon finds herself in a love triangle between Night and her childhood friend Soshi, who declares his love for her out of fear he will lose her. As the series progresses, Night begins to develop real human emotions, enabling him to truly love Riiko but also resulting in system malfunctions. When Riiko almost loses Night due to the malfunctions, she realizes that he is the one she really loves. She apologizes to Soshi, who moves to Spain with his brother to live with their dad. Night and Riiko spend a few happy weeks together, during which they go on dates, get their picture taken together, and purchase matching rings as symbols of their undying love.
As the series ends, Night begins to grow sleepier and sleepier. The problems developed by him exceeding his abilities eventually causes his machinery to stop working, resulting in his "death". Before he died, he wrote a letter directed to Soshi telling him what was happening and asking him to take care of Riiko. He also sends his ring, which Riiko noticed he has not been wearing immediately prior to his death. In the letter he tells Riiko that she doesn't have to cry for him any more. Riiko then smiles through her tears and forever treasures the ring, picture and her memories of Night her first love, and eternal boyfriend.