The Silver Chair is the fourth of the Narnia tales (in the order that they were published) and tells the story of the rescue of Prince Rilian, King Caspian's son. This story features Eustace who readers of the series met in the Voyage of the Dawn Treader and his friend, Jill Pole.
The story begins with Eustace and Jill running from some bullies. Eustace had been one of the bullies, but his prior experiences in Narnia had changed him, and his friends and enemies had noticed and had changed themselves as well. After Eustace and Jill found themselves in Narnia, Eustace told her about his previous visit to Narnia, and suddenly a door in the wall transports them to a huge cliff. Jill dares Eustace to go to the edge of the cliff and he falls off. Aslan blows him off to Narnia and then tells Jill why they have been summoned. They are to go find the missing Prince Rilian. Aslan then gives her four signs to remember in order to find the prince. The first sign is to look for an old friend. Aslan then blows Jill to Narnia behind Eustace. When she arrives next to Eustace they are watching a ship embark from Narnia with an aged king on board. They soon realize that the aged king is Caspian and he is the friend they should have contacted, but they missed their chance. They will later miss two more of the four signs.
Because of this missed sign, the children go on their quest not with supplies or an armed escort, but make their own way with an unusual escort for their journey: a Marsh-wiggle named Puddleglum. The name suits him, with his overly pessimistic outlook on life, but he soon becomes a trustworthy and brave companion to the children. The children learn that Rilian's disappearance is connected to the fact that his mother had been murdered by a serpent. While Rilian was searching for her, he was taken by an enchantress who was most likely also the serpent. The children and Puddleglum go off to the north to a land of dumb, evil Giants.
Later they come to a land of the gentle Giants and the city of Harfang where they are warmly welcomed. They are invited to stay and are promised a marvelous time at the Autumn Feast. Soon they find to their horror, that "man" is one of the dishes eaten at the feast. They immediately begin planning their escape which goes well until they are outside the city and are seen. They hide in a hole in the ground and suddenly find themselves falling into an underground country where they are captured by gnomes. They are taken further underground where they are introduced to the Lady of the Green Kirtle who rules the underground land and her knight. The Knight turns out to be Prince Rilian, but he does not remember who he is. However, Rilian remembers his past for a short time each evening when he is bound by the Lady's orders. The children hear him in his supposed ravings and he asks them to release him in the "Name of Aslan".
Those words are the fourth sign Aslan had told Jill to seek. The children then release Rilian and later kill the Lady of the Green Kirtle who changes enraged into the green serpent that killed Rilian's mother. They then find that her plan was to bring an army out of the ground into Narnia and place Rilian on the throne as a puppet king. The children find the gnomes were from an even lower underground land named Bism and free them to return home. They then return to Narnia where Rilian is greeted with celebration and he is briefly reunited with his father Caspian before he passes. The children return with Aslan and Caspian to Aslan's country which seems to be the equivalent to Heaven as Caspian regains his youth there. Eustace and Jill return to their school with Aslan and Caspian and defeat the bullies with the broad sides of their swords, changing the school forever. Then Caspian and Aslan return to their own land.
Quotes from The Silver Chair by C.S. Lewis