The Silver Chair, a children's fantasy novel, is the sixth installment of the Chronicles of Narnia. The story recounts the adventures of schoolchildren Eustace Scrubb and Jill Pole after they cross over into Narnia. They are tasked with finding Prince Rilian, who has been missing for several years. On their quest they have many adventures, including meeting Marsh-wiggle Puddlegum and the Lady of the Green Kirtle. By closely following the advice of Aslan, they are able to find and rescue Prince Rilian.
The story starts when Eustace Scrubb, introduced in The Voyage of the Dawn Treader , is driven into the company of classmate Jill Pole at their miserable school Experiment House. The impetus is their need to find sanctuary from the gang of school bullies who run rampant in this laissez-faire and mismanaged school run by an incompetent headmistress. Eustace confides to Jill that he has recently been "out of this world" to a land called Narnia, and that his experiences there have led to the changes in his behaviour that everyone seems to have noticed. Jill initially thinks that Eustace is lying, but when he promises and asks her to attempt to go to Narnia with him, she agrees. When the bullies are about to converge on the two, Eustace suggests asking for Aslan's help, and the two blunder through a gate that leads them to a high cliff in Aslan's Country.
Jill shows off by approaching the cliff's edge, and Eustace, trying to pull her back, falls over the edge. Aslan appears and saves Eustace by blowing him to Narnia. He charges Jill with helping Eustace find Prince Rilian of Narnia (the son of King Caspian X), who disappeared some years before, and he gives Jill four Signs to guide her and Eustace on their quest. The fourth and final Sign is that at a key moment they will be asked to do something in Aslan's name, and they must do it. Aslan then blows Jill into Narnia, where she arrives a few moments after Eustace—just as an elderly and frail King takes ship and sails from the harbor. Jill remembers the Signs, and asks Eustace if he has seen an "old friend"; if he had, and had greeted him, they would have received help in their quest. They eventually realize, much to Eustace's chagrin, that the departing King is actually King Caspian X, who has set off to search for Aslan or for his son. Trumpkin the Dwarf, now Lord Regent and quite 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. Master Glimfeather summons themto a Parliament of his fellow talking owls. The owls explain that Caspian's son, Prince Rilian, disappeared while searching for the green serpent that killed his mother, and is under the spell of an enchantress.
Jill and Eustace are flown to the marshes on the northern edge of Narnia where they are partnered with the delightfully gloomy but stalwart Marsh-wiggle Puddleglum, who serves as a guide, hunter for food in the wilderness, and down-to-earth voice of reason. They journey toward the giant-lands north of Narnia after crossing the River Shribble. The first giants they encounter take no notice of them, and the trio continue north where they encounter a deep and sinister canyon. The only route across this barrier is an enormous and ancient bridge, many times larger in scale than anything a human might normally use.
Hungry and suffering from exposure, they cross the bridge and 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". There, she tells them, they will find warm beds and hot food while the Giants celebrate their Autumn Feast. Jill and Eustace overcome at the thought of comfort and warmth, forget all about the signs and the quest; only the straight-headed Puddleglum argues against the journey to Harfang. They continue traveling north for several days, finally noting the lights of what they believe to be Harfang one evening. The next morning, they proceed to Harfang, but are waylaid by an immense snowstorm as they cross an oddly-shaped hill covered with ruins barely noticeable in the driving snow. Jill stumbles into a mysterious chasm that is shaped like a letter "E". At that point, the snow starts to clear and they again notice the lights of Harfang. The company hastens there lest the doors be locked (which the Green Lady had warned them would occur should they arrive too late in the afternoon).
Upon their arrival, they find the Gentle Giants only too pleased to "have them for their Autumn Feast." After a night of rest, the three look out a window of the castle and discover the obvious ruins of a giant city in the valley below—which they had passed through but failed to notice due to the snowstorm. They also see the words "Under Me" engraved on the road, which they recognize as Aslan's third Sign. Later they make another discovery: a cookbook that describes how to prepare and serve various beasts, including men and Marsh-wiggles; the giants are planning to eat them for the Autumn Feast. After finding an unguarded door, Scrubb, Pole, and Puddleglum escape the castle only to be chased by hunting dogs and giant nobles. They take shelter in a cave under the ruined city, where they fall down a long dark slope into Underland. Battered and bruised, they are now in complete darkness; but they have followed the Sign that said "Under Me".
They are found by an army of earthmen, who take them aboard a boat across a Sunless Sea to the city ruled by the Lady of the Green Kirtle. Her protégé, a young man, greets the travellers pleasantly but does not seem right in the head. He explains that he suffers from nightly psychotic episodes, and during these episodes he must, by the Lady's orders, be bound to a silver chair; for if he is released, he will turn into a deadly green serpentand kill everyone in sight. The threesome determine to witness the youth in his torment, as they sense it could be the key to their quest.
When the young man is tied to his chair, his "ravings" seem instead to indicate desperation to escape an enchanted captivity. After launching a battery of dire threats, the youth finally begs his companions to release him in the name of Aslan . Recognizing the fourth Sign, they hesitantly do so. Far from their apprehension of him turning into a serpent or killing them, the young man proceeds to destroy the silver chair. Free from the enchantment, he thanks them. He declares that he is the vanished Prince Rilian, kept underground by the Lady of the Green Kirtle as part of her plot to conquer Overland.
The Green Lady returns and tries to bewitch them all into forgetting who they are. The barefoot Puddleglum stamps out the enchantress's magical fire and breaks her spell. The enraged Lady transforms herself into a green serpent, and Rilian realizes that he has been enslaved all these years by his mother's murderer. Rilian kills the serpent with the help of Eustace and Puddleglum, and leads the travellers to escape from Underland. The gnomes, who had also been magically enslaved by the Lady, are now freed by her death and joyfully return to their home even deeper in the earth, a land called Bism. One of them shows Rilian's party a route to the surface before leaving. Rilian returns to Cair Paravel as King Caspian is returning home, and Caspian is reunited with his long-lost son just before dying.
Aslan appears and congratulates Eustace and Jill on achieving their goal, then returns them to the stream in his country where Jill first met him. The body of King Caspian appears in the stream, and Aslan instructs Eustace to drive a thorn into the lion's paw. Eustace obeys, and Aslan's blood flows over the dead King, who is revived and returned to youth. Aslan explains that when Jill and Eustace return to their own world, Caspian will go with them briefly, to help set things right there. At the portal between the worlds, Aslan roars, and part of the wall surrounding Experiment House collapses. Caspian, Eustace, and Jill cross the wall and give the school bullies a sound thrashing.
The beaten bullies run back towards the school in terror, having also seen Aslan, who lets them glimpse his back as part of the plan. In the confusion Eustace and Jill sneak back into the school building and change into their school clothes, while Aslan and Caspian return to Aslan's country. The headmistress calls the police with a wild story of armed hoodlums and an enormous lion on the school grounds; subsequent enquiries expose her incompetence and mismanagement. The worst of the bullies are expelled and the incompetent headmistress given a new job—failing as a school inspector, she is eventually elected to Parliament. Experiment House becomes a well-managed learning institution, and Eustace and Jill remain good friends.