The Silver Chair (The Chronicles of Narnia)

"Quick!" said Eustace. "Hold hands! We mustn't get separated!" And before Jill quite knew what was happening, he had pulled her out of the door, out of the school grounds, out of England, out of our whole world into That Place.

Eustace and Jill are whisked to the land of Narnia where Aslan, the great Lion, needs their help to find the missing Prince Rilian. Teaming up with Puddleglum, the Marsh Wiggle, the search takes them through some of the most dangerous underland of Narnia. Even if they attain their goal, it can only be the start of further trouble.