The novel takes place in a fantasy world during medieval times. Magic is apart of society, but not common among normal people, and it can manifest itself in a lot of different ways - including as a disease. One disease in particular is pretty nasty, and has a track record of killing entire civilizations if not contained. The results of it aren't pretty either, and while it appears with the symptoms of the flu, it ends in the victim either dying or transforming into a monster. If a person is bitten, they've also a likely (if not guaranteed) chance to catch it.
The actual story begins with a farmhand named Asline, who's completely uneducated and only knows how to barter crops for a living. She lives in a paranoid, walled-off kingdom which is strict about who it lets in and kicks people out on a daily basis for illegally immigrating. Because it's so very isolated, not much happens and its military is quite lax. Once reports of a certain disease start appearing however, the nobles panic and an order to evacuate the sick is issued. Unfortunately the order came too late, and a servant of the royal princess shapeshifts into a monster, and injures the princess herself.
Overwhelmed with grief and anxiety, the monarch decides enough is enough, and the search for a cure begins. Ironically, they find that a certain stone within their possession said to contain the blood of their patron saint is what staves off the illness. Sadly, over the many years the crimson stone has slowly withered away and it's not enough to completely cure the princess or anyone else. From there it's decided that they must find another source, and there's a rumor one can be found across the continent in the ruins of an ancient civilization.
Because the shadow beasts still lurk beyond their wall, the knights of Oscera are outfitted with both a chip from the sacred stone, and given their choice of a monster to take with them as an enslaved attack dog. Most of the time they find their monsters from citizens right within the walls; pitting the sickly against each other in an arena where they fight until they transform - the surviving beast awarded to the departing knight.
Months later, very few of the knights return alive and none are successful. The princess degenerates slowly as the crimson stone merely prolongs the inevitable. Frustrated, her first and personal knight Jeremy decides to leave and look for the cure himself even though so many haven't returned. At the same time, Asline and her grandmother attempt to flee the kingdom and avoid the raging plague. The night before their escape however, Asline comes down with a fever and they're both caught. She's then forced to fight in the bloodied arena against shadow monsters even though she feels sound of mind. After one long grueling beating, she finally gives in to her darkest thoughts - wanting to die - and she transforms, crushing multiple opponents. Having watched the arena fights, Jeremy spots her and intends to choose her and his accompanying monster. What he finds the next morning in the cell however, is a young girl beaten and bruised badly. Somehow, Asline had pulled herself back into a human being.
The rest of the story is essentially Asline coming to terms with her low self-esteem and self-image, as well as finding a cure for the plague. Mostly though, it's about learning to like yourself - because the more Asline transforms into her monster form, the more deformed as a human she becomes, and thus the more she hates herself. The darker her personal hate and thoughts are, the more powerful she becomes. In the end she has to decide whether to sacrifice her own self-worth and become the most powerful shadeshifter, or return to being herself again and leave the rest of her kingdom in the lurch.
It's essentially a garbled mix of Disney fairy tale gone wrong combined with a gender-swapped Beauty and the Beast/Phantom of the Opera/Hunchback of Notre Dame/The Man Who Laughs, in which instead of the man being disfigured in some way it's the girl.
