Rosemary's plan to clean houses during her summer break and surprise her mother with the money hits a snag when an old lady at the market talks her into buying a second-rate broom and a cat she can't even afford to keep. But appearances can be deceiving. Some old ladies are witches, some brooms can fly, and some ordinary-looking cats are Princes of the Royal Blood. Rosemary's cat ("You may call me Carbonel. That is my name.") soon enlists her help in an adventure to free him from a hideous spell and return him to his rightful throne. But along the way Rosemary and her friend John must do some clever sleuthing, work a little magic of their own, and - not least - put up with the demands of a very haughty cat.