A Hat Full of Sky revisits the young witch Tiffany Aching, who is preparing to leave home and learn witchcraft. Working with Miss Level to learn the use of plants, Tiffany becomes worried by a mysterious presence in the cottage. It turns out to be Oswald, an ondageist which is a sort of reverse poltergeist, a spirit that is an obsessive cleaner. She takes dislike to her studies, finding the work of the witches to be thankless, dull and pointless. They work long hours helping the sick, young or elderly who are not self-sufficient. A hiver, a mysterious and dangerous creature, enters Tiffany's mind causing her to abuse her powers of magic. It is explained that the Hiver does not change the way she acts, merely allows her to do what she would do without a conscience. Her fellow witches are worried by Tiffany's strange behavior and try to bring her back to herself.
The Nac Mac Feegle arrive to help Tiffany and, along with Miss Level, enter Tiffany's mind in order to drive off the Hiver. With the help of Mistress Weatherwax, Tiffany traps the Hiver and learns its true purpose, which is to seek shelter from the world, where it is aware of everything, or, as Tiffany calls it 'opening your eyes and opening them again'. Tiffany eventually leads it to Death where it finally crosses the desert to the afterlife.
At the end of the novel, Tiffany returns to her home to take the place of her grandmother as witch for the people. A witch's hat is very important, because it lets other people know that you are a witch. However through the story it is revealed Tiffany does not need the traditional hat that she had been wearing as the sky forms her hat.
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