Aileen was supposed to become a wise woman like her Aunt Beck, but then she messed up her initiation and spends the following morning in one of those depressions that sucks the joy right out of eating. So maybe it's just as well that she and her aunt are summoned to the castle and sent off on a quest to reopen the sealed land of Logre, which vanished behind a wall not long after Aileen was born. Things go wrong. When Aunt Beck gets herself cursed out of her own stubbornness, it's up to Aileen to take over and deal with all the problems — both large and small — that crop up.
When Diana Wynne Jones died in 2011, we lost one of the great fantasy and children's writers of the past century. There's a sense, not just of magic and quests, but of people who actually have to live in a world where curses might mean that every meal has to be spoon-fed, and where the horse isn't a gallant steed but a donkey that gets the cart stuck in the mud sometimes. There's true love, the refutation of childhood crushes and a gentle understanding that people sometimes make the wrong choices when they're alone. It doesn't take center stage, but there's a lot that an older reader will get that a child won't.
What makes a Diana Wynne Jones story is the understanding that as wonderful as magic is, it doesn't solve problems. Magic is merely an extension of the personality of the people who use it. Character, not power, decides the fate of Aileen and her companions.
The Islands of Chaldea was completed by Diana Wynne Jones's sister, Ursula Jones, an acclaimed novelist and actress.