Eustace Scrubb, now a reformed character following the events of The Voyage of the Dawn Treader, encounters his classmate and new friend Jill Pole at their school, Experiment House, where they are miserable. Jill has been tormented by bullies and is hiding from them. Eustace tells Jill about his Narnian adventures, and how his experiences there led to the changes in his behaviour – which Jill warns is likely to see him targeted by the bullies as well. Eustace suggests asking for Aslan’s help, and as the bullies converge on them, the two blunder through a gate that leads them to Aslan’s Country.

They encounter a cliff, where Jill shows off by approaching the edge, and Eustace, trying to pull her back, falls over the edge. Aslan appears and saves Eustace by blowing him on a magical wind stream to Narnia. He charges Jill with helping Eustace find King Caspian X’s son, Prince Rilian of Narnia, who disappeared some years earlier. He gives Jill four Signs to guide them on their quest and then blows Jill into Narnia, where Eustace is already waiting by a great castle. They watch as an elderly and frail man takes ship and sails from the harbour. To Eustace’s dismay, they learn that the elderly man is actually King Caspian; by failing to greet him they have missed the first Sign. Seventy years have passed since Eustace was last in Narnia, even though less than a year has passed in his world. They also learn that Caspian has sailed off to visit again the lands they had sailed to when he and Eustace were young, although many Narnians believe that he has set off to seek Aslan in order to ask who can be the next King of Narnia when he dies. Caspian is obviously deteriorating with old age, and his people fear that he will not live for much longer.
Caspian’s Lord Regent Trumpkin the dwarf, now very elderly and deaf, provides Jill and Eustace with rooms in Cair Paravel, but on the advice of Glimfeather the Owl, they make no mention of their quest. Glimfeather summons them to a Parliament of his fellow talking owls, who explain that Prince Rilian disappeared a decade earlier while searching for a large green serpent that had killed his mother.

Jill and Eustace are flown to the marshes on the northern edge of Narnia where they meet their guide, Puddleglum, a gloomy but stalwart Marsh-wiggle. They journey toward the giant-lands north of Narnia. Hungry and suffering from exposure, they meet the Lady of the Green Kirtle accompanied by a silent knight in black armour. She encourages them to proceed northward to Harfang, the castle of the “Gentle Giants”, who she says would be glad to have them at their Autumn Feast. Jill and Eustace, overcome at the thought of comfort and warmth, are eager to go; only Puddleglum argues against the journey to Harfang. After a long journey in harsh weather, and braving a mysterious chasm in a driving snowstorm, they are welcomed at Harfang.

From the castle the three see that in the snowstorm they had blundered through the ruins of a giant city in the valley below, thereby missing Aslan’s second Sign. They also see the words “Under Me” engraved on the road, which is the third Sign. Discovering from a cookbook in the kitchen that they are the main course for the Autumn Feast, they make a narrow escape from Harfang. Following the Sign, they take shelter in a cave under the ruined city, where they fall down a long dark slope into Underland.