A high school student named Itsuki Tomonari finds his life derailed when his gambling-addicted parents abandon him, leaving him penniless and facing the real possibility of dropping out of school. His fortunes take an unexpected and dramatic turn when he is kidnapped alongside Hinako Konohana, the sheltered heiress to one of the largest conglomerates in Japan. After this harrowing encounter, Hinako quickly grows attached to Itsuki, leading to a life-changing proposition. He is hired to become her secret caretaker and moves into the Konohana estate, transferring to her prestigious and wealthy school, Kiou Academy.

The central conflict of Rich Girl Caretaker stems from a stark public versus private dichotomy. To the outside world, Hinako is the perfect young lady brilliant, elegant, and admired by all her peers. In truth, she is lazy, possesses almost no basic life skills, and relies on others for daily tasks she cannot perform herself. Due to intense pressure from her influential family to maintain this flawless public image, she must keep her real, helpless nature a secret. Itsuki becomes the only person who knows the truth, devotedly managing her needs from preparing her meals to organizing her schedule all while hiding his role as a caretaker from their classmates.

The story is set within the glittering but high-pressure world of Kiou Academy, a school exclusively for the children of the ultra-wealthy. Itsuki must navigate this rigid social hierarchy as a commoner, balancing his part-time jobs with the demands of covertly supporting Hinako. The school setting introduces a cast of classmates from other powerful families, including Mirei Tennoji, the scion of a rival group who considers herself Hinako's rival, and the shy Narika Miyakojima. Itsuki is also trained by Shizune Tsurumi, a cool and strict maid of the Konohana family, and his childhood friend Yuri Hirano keeps an eye on him from afar.

The narrative unfolds across several key arcs that blend romantic comedy with slice-of-life drama. The story begins with the initial arrangement, where Itsuki struggles to learn his duties while Hinako secretly delights in his attentive care. As they settle into a routine, they must avoid the suspicion of their classmates, including Karen Asahi and Katsuya Taisho, who initially pose as commoners but are themselves from powerful industrial families. The evolving relationship between Itsuki and Hinako forms the emotional core of the story. What begins as a master-servant relationship based on obligation gradually deepens into mutual affection, with Hinako becoming emotionally dependent on Itsuki and expressing that she cannot be without him. This growing bond transcends their employer-employee dynamic, forming a heartfelt love story amidst the backdrop of elite society, family expectations, and the simple, comedic trials of managing the daily life of a helpless but endearing heiress.