Kyle Kingsbury is a selfish, shallow high school freshman who thinks that his money and social standing earn him the right to behave in any way he wishes. His behavior, as pointed out by a Goth girl at school, is "beastly." After Kyle enacts a plan to humiliate the Goth girl who upset him, he is transformed by a witch into a horrible beast. He has fur, claws, fangs and, in the beginning, a bad attitude. Kyle must now learn to live with himself as a beast and must make changes in order to be happy. The witch has given Adrian two years to break the spell she has put on him. If he can learn to love and be loved in return, he will change back into the the boy he was.

Kyle's father abandons him in a five-story brownstone in Brooklyn, where is is cared for by his maid, Magda, and his blind tutor, Will. It is with Magda and Will's help that Kyle grows into a better person. Once his personality starts to change, he becomes ashamed of the person he was. He takes a new name, Adrian, and begins to learn how to appreciate the real, simple things in life. Kyle, as Adrian, begins to grow a rose garden which helps him to see the beauty in life.

Adrian resigns himself to being a beast forever until a chance encounter with a thief changes his life. The thief offers to give Adrian his sixteen-year-old daughter in exchange for not having him arrested. Adrian agrees and the girl, Linda, is brought to live in the house with Adrian, Will and Magda. At first, Linda is angry and feels like a prisoner, but as the two get to know each other they develop a bond. Adrian and Linda become friends as they share a love for flowers and learning. They also seem to understand each other on a level they didn't before. Adrian begins to feel guilty for the arrangement he made with Linda's father, an arrangement that makes her a virtual prisoner. Adrian falls in love with Linda and decides to let her return to her father, whom she is concerned about. Linda says she will visit him, but Adrian knows in his heart that she won't.

The two are separated for many months. During that time Adrian is depressed. One night Linda is in danger and calls for Adrian's help. He heedlessly runs to help her and is almost caught in the process. After saving Linda, the two express their love for each other and kiss. The kiss changes Adrian back into Kyle. Lindy is uncertain, but loves Adrian no matter who he is. The two return to the brownstone to find Will and Magda and live happily ever after.