Mary discovers a secret room built under the Academy. When she and Prince Reifus go to investigate it, they discover a box containing a circlet. At the behest of a mysterious voice, Reifus puts it on, causing him to transform from a handsome prince...into a beautiful princess?! Feeling responsible for her involvement in the incident, Mary tries to resolve the situation—but fixing this won’t be easy!
Even though there are two more books in this series, my time with this series must come to a temporarily close. Unfortunately I do not know Japanese, and this book here, which just released an English edition, is the furthest in the series that has been translated.
The main character, the Invincible Little Lady, aka Lady Mary Regalia daughter of a duke, has her story continue. Her group of friends has been tasked to find abandoned experiments and . . . do something about them. The school headmaster and the other adults in charge are quite lazy and lax in overseeing what the kids do, and many have done random experiments and just left them as is somewhere on campus. So Mary's group opens the book trying to find said abandoned experiments and deal with them.
With the help of her friendly kitty-cat, Mary and Reifus [spelling probably wrong] find a hidden chamber buried underground. The Prince finds a piece of jewelry in a box and is compelled to put it on. Whereupon he becomes a she. The rest of the book follows the group as they attempt to find a way for the prince to stop being Princess Rain (so named by her mother) go back to being a Prince. They travel first to a specific village and a nearby manor/castle/thingie where the boy who made the device should currently be residing. While there they encounter vampire(s). Learning that they need to find the 'helper' who helped the boy make the device, as they are the ones who actually made it not the boy, the group wanders over to a forest and encounter elves and giant rats.