Onii-chan Rehabilitation Project

Onii-chan Rehabilitation Project (known as the Big Brother Rehabiliation Project in the anime) is a program developed by Mihari Oyama to help her older brother Mahiro reintegrate into society.

Origins
Mihari Oyama has always thought the world of her older brother Mahiro, and the two siblings were incredibly close as kids. The two always bathed together, Mahiro won Mihari stuffed animals at amusement park games, he bought her stickers when she became interested in them, and he even gave her rewards, like her signature hairclip, for doing well on tests. The latter in particular pushed Mihari to excel so she would receive more love and affection for her brother, and she soon developed into an absolute prodigy at everything she did. She broke records in middle school sporting events that remain unbroken to this day, and eventually ended up repeatedly at the top of her class.

Unfortunately, Mihari's success had a detrimental effect on Mahiro. Due to her excelling despite being younger than him, people would start to compare his lack of success to Mihari's success, which aggravated an inferiority complex that he felt in comparison to her. As a result, Mahiro stopped making time for Mihari and began isolating himself in his room, doing nothing but reading hentai manga and playing video games. He even stopped leaving the house altogether after a certain point.

On Mihari's first day of high school, she was delighted by how she looked in her uniform and went to Mahiro's room to get him to see her in it. By this point he was already isolating himself with a 'KEEP OUT' sign on the door, and received no response when she tried to talk to him. Mihari started to get worried about Mahiro, as she missed him and wanted to find a way to help him.

While walking to school, Kaede made an offhand remark to Mihari about how using makeup and wearing fashionable outfits can make someone look good and improve their self confidence. This gave Mihari the idea to turn Mahiro into a girl. She then realized that if there was no opportunity for progress, there would be no results. Thus, she got the idea to de-age Mahiro as well. From that moment, her goal was clear: to turn Mahiro into a young girl to reintegrate him into society.

Realizing that she needed funding and research in order to do this, she took several tests that allowed her to skip high school and go straight to college. There, she focused on biology and chemistry where she started the project by developing a drug that would turn Mahiro into a young girl.

The project eventually caught the eye of Mihari's faculty advisor Chitose Azuma, who made key contributions to perfecting the drug. Although Mihari believed that she did this out of the goodness of her heart, in actuality Chitose has a secret agenda that she wants to see carried out through the project as well.

The night before the events of "Mahiro's Confusing New Body", Mihari slipped the drug into Mahiro's food (drink in the anime) to commence with the project. That night as Mahiro slept, the drug transformed him into a middle-school aged girl.

Overview
The goal of the Onii-chan Rehabilitation Project is simple: to reintegrate Mahiro back into society by transforming him into a young girl and subsequently making changes to his routine so he doesn't revert back to how he was.

The first stage of the project was getting Mahiro into a new routine with her new body so she would be cleaner, have more confidence in herself, and get used to being a girl. The first major change that Mahiro experienced was an inability to become aroused by the eroge games she fancied so much as a boy, things that Mihari saw as key obstacles to Mahiro's growth as a person. She got Mahiro into the habit of showering daily, brushing her teeth, and washing her face to become cleaner, as she had previously gone days without showering. She also began to lay out girl's outfits for Mahiro to wear each morning so she would get used to dressing herself and become more confident in her appearance. To prevent further backsliding, Mihari claimed that she had gotten rid of all of Mahiro's male clothing, though in actuality she just stored it away in his closet. She did these things because she knew that Mahiro otherwise would never change clothes. At the same time, she guided Mahiro through her first period and taught her how to use pads and tampons to deal with it in the future. Eventually, Mahiro got used to this routine and began washing and dressing herself without Mihari telling her to do so.

Mihari also attempted to get Mahiro into an exercise routine, but abandoned it after Mahiro refused to go out again after the first attempt. The attempt did however lead to Mahiro leaving the house for the first time in two years, and opened up the door for her to start leaving it more often.

The next phase of the project involved socialization with others. Mahiro had previously had no friends, and had never been in a relationship with anyone outside the house. Also, Mahiro's being isolated for two years made her afraid of large crowds. Thus, Mihari formulated a plan for Mahiro to connect with others and get used to being around people. This began when she invited her friend from middle school Kaede Hozuki over to the house one day. Although Mahiro was nervous about her being there, and that she might figure out the truth about her, she really bonded with Kaede. Mahiro was amazed by Kaede's cooking skills, and was equally joyed by how open and helpful she was. By the end of the visit, Mahiro started to see Kaede as another big sister figure in her life and looked forward to seeing her again.

Mahiro began to see more of Kaede as the weeks progressed and continued to grow close to her. All three of them met up for a movie, where they went shopping beforehand. During the movie, Kaede found Mahiro in the bathroom and helped her get new underwear after she wet herself while waiting for a stall, and agreed to keep it a secret. Mahiro even called Kaede for advice on how to make rice porridge when Mihari fell sick.

Mahiro's relationship with Kaede soon allowed for her to meet other people that she would grow equally close to. By chance, Mahiro encountered Kaede's younger sister Momiji while shopping at the supermarket, and the two quickly struck up a friendship. Momiji began accompanying Kaede when Kaede visited the Oyama household so she could see Mahiro again, and the two quickly became best friends. The four of them met for a Halloween party, and even took a trip to a hot spring.

After several weeks, Momiji decided to introduce Mahiro to her two best friends from middle school Asahi and Miyo. The two immediately hit if off with Mahiro, and the four girls became an inseparable group of friends. After everything Mahiro had been through, she finally had a functional social life.

With Mahiro having a tight-knit social circle, Mihari decided to put the next phase of her plan into action, which was enrolling Mahiro back in middle school. The hopes with this was to allow Mahiro a chance to start over and feel better about herself and get more comfortable around larger groups of people while attending alongside her new friends, as well as opening up the chance to expand her connections. Mahiro was enrolled in the same middle school as Momiji, Asahi, and Miyo (which was also the same middle school she had once attended herself as a boy), and was placed in the same class as them; Class 1-B. All this was done courtesy of Mihari, who forged identity documents with the help of Chitose Azuma and made up a cover story for Mahiro that could plausibly explain her previous absence.

Although difficult at first, Mahiro quickly adjusted and began living the life of a middle-school girl. She formed connections with her classmates and continued to remain close to her new friends. Shared experiences of hardship brought her closer to Satsuki, Minori, Mai, and Miyako, while acts of kindness and shared interests in video games earned her the attention and attraction of Yuta and Minato.

By the start of the new year, Mahiro and her classmates moved to Class 2-B, and a new student Nayuta Tenkawa joined them. Nayuta revealed that she knew the truth about Mahiro, claiming to be Mihari's research assistant. Unknown at first, Nayuta was the ward of Chitose who asked for Nayuta to keep tabs on Mahiro so she could spy on Mahiro without Mihari's knowledge. Soon after, Asahi invited Nayuta into their group of friends, and they all became inseparable.

At this point, it is unclear what the end goal for the project is intended to be. Mihari has continued to provide Mahiro with extra doses of the drug so she can remain female, but it is unclear as to what she intends for this to lead to. It is unknown if she intends for Mahiro to grow up fully as female, or if she intends for her to one day return to being a guy with the new experiences making him a better person.

It is also unclear as to why Chitose is so interested in Mahiro and her progress.

History
The history of the project is in essence the events of the series as a whole. This is further emphasized in the English translation as each chapter is labeled as 'Step', making each chapter another step of the project that Mahiro is experiencing.