The Silver Chair
By C.S. Lewis
Jill and Eustace travel to Narnia where Aslan gives them the task of searching for and rescuing a prince. Through many adventures, the children are accompanied and guided by Puddleglum, a frog-like swamp-man. Jill has been instructed by Aslan to remember certain signs. However, the group's limited ability to stick to the signs leads to a number of difficulties they have to overcome. In the end, they manage to rescue Prince Rilian who slays the wicked witch and save Narnia.