In an effort to escape bullies at her horrible English boarding school, Jill Pole runs into the school gardens as far as she can and tries to hide. She is discovered by a boy called Eustace Scrubb who used to be one of the snitches for the bullies but who had recently changed and started to stand up to them in the defense of others. He tells Jill that this is because he travelled to a land in the other world where he went to a place called Narnia and met animals who could talk; he fought alongside a Narnian prince and became a changed young man. As the bullies approach, Jill and Eustace find that a gate that is always locked is miraculously unlocked and they escape just in time, finding themselves atop a sheer cliff that Jill is unafraid of.
Trying to stop Eustace from pulling her away from the edge, Jill accidentally pushes him over it. Alone now she sees an enormous lion who speaks to her and tells her that he brought her and Eustace to Narnia to do a task for him. That of finding the missing Prince Rilian who is believed to have been kidnap led. He tells her that they are to keep searching until they find him, or die trying, or return to their earthly world. He also tells her that there will be signs along the way that they are on the right path; Eustace will r recognize the first person he speaks to, they will see stone lettering before they get to an old giant's castle and someone will ask them to do something in the name of Aslan. Aslan then blows her gently on his breath to land where she sees Eustace again. He discovers the prince he sailed with the year before is now an old, old man because Narnia time is different to earthly time.
Having already messed up the first sign because Eustace arrived before Jill and didn't know there were any signs in the first place, they retire to sleep under the watchful gaze of Glimfeather, leader of the owls. They help the children steal away in the dead of night and they are soon accompanied by Puddleglum, a Marsh-Wiggle with a gift for always seeing the worst case scenario. The prince they are looking for is Prince Rilian and as a young boy his mother was murdered by an evil serpent by a beautiful fountain. Wanting to avenge her death Rilian spent most of his time at the fountain waiting for the serpent who didn't return, but whilst at the fountain he saw the most beautiful woman he had ever seen before. One day he vanished and since then although many have searched for him nobody has found him. Now as King Caspian nears death, finding Rilian is all the more urgent.
Their trip is long and tiring and also rather cold because Narnia and the northern lands are snowy. On their travels they meet a beautiful Queen and her companion, a knight who is totally hidden by his own armor. The children are quite open with her but Puddleglum is more circumspect because they don't know her and he can feel that there is enchantment in the air. The Queen tells them to go to the giant's castle and tell them that she has sent two children and a marsh wiggle for the feast. Hungry and chilly they are excited to hear this and continue on towards the castle she has described.
The terrain is difficult and it is dark; they find themselves in a cold, rock-like maze that seems to be full of dead ends and wrong turns. Eventually they reach the giants' castle and give the porter the Queen's message. They are received rapturously and given food and a warm bed. The following morning when they look out of the castle window they see huge stone letters that spell the phrase "I'M UNDER HERE". They realized that they have messed up the second sign two and that the maze they thought they were walking in was actually the letters. They realize they need to get away but have also come to realize that they are prisoners in the castle and that rather than being guests at the feast they are going to be served at the feast. They concoct an elaborate plan to escape which works for a short while but their absence is noticed and soon the giants are on hot pursuit. As they are giants they cover the ground a lot faster than two children and a marsh-wiggle but in the nick of time the three escapees slide into a hole and hastily block it up with a stone so that the giants cannot follow.
Jill, who is claustrophobic and frightened of the dark, is absolutely terrified. They slide by accident several feet lower and find themselves face to face with a rather unfriendly group of guards. They are taken to a room where they see a man who is all together more welcoming as he says he has met them already on the enchanted bridge; they realize he must be the knight whose face was obscured. He is hospitable and invites them to share his sumptuous dinner which they do with enthusiasm. He tells them that he has no memory of his life before being with his Queen and his total allegiance is with her. She is grooming him to infiltrate a country who will welcome him as their king, believing him to be a good and benevolent ruler, but he will be opening the door for the Queen to take over in what sounds to be more like a dictatorship than a kindly monarchy. The children are privately rather disgusted and don't like him much after all. Puddleglum is a little more outspoken. The knight tells them that he will shortly be tied with ropes to his chair because there comes a time every evening when his demeanor changes; he hollers and screams and makes all sorts of threats so for his, and others', safety, he is restrained. He tells them that he will most likely try to persuade them to untie him but they should not do so. Amongst themselves they vow that under no circumstances will they unite him. He begins to change, to wheedle and beg and shout but they resist. Then he asks that they release him in the name of Aslan.
Realizing that this is the sign Aslan told them about they nervously untie him and at once he is calmer. He remembers Narnia and that he is actually Prince Rilian, and that he was kidnapped by the Queen. He also realizes that the country she planned he should take over was Narnia itself. He readies himself to leave but with very unfortunate timing the Queen arrives and is angry to find that he recalls who he is. She throws strange aromatic powder on the fire which at once makes them all sleepy and suggestible. Puddleglum resists the enchantment which gives the others the power to do so as well. Angry, the Queen turns into a serpent. Edited their eyes and Rilian realizes he has been living with the serpent who murdered his mother. Between the he and Eustace overpower the serpent and cut off its head.
As they escape they are pursued by a legion of angry-looking gnomes. Puddleglum catches one of the smaller ones.Hid name is Folg and they all realize they are all on the same side; the gnomes were condemned by the Queen to a life far too near the surface than they can bear and they have been waiting for their opportunity to go back down deeper underground. They agree to lead the Prince and his new friends to the entrance to the Overworld that they have been constructing. It is yet another dark and narrow route for Jill to tolerate. Seeing a light above them they decide that Jill should have a look out of the top. She disappears suddenly and seems to have been snatched from above. To Jill, this is not frightening at all because she realizes they are happily back in Narnia. To the remainder of the group it is very worrying because she seems to have been kidnapped upwards. Because of this when the elfs and the moles dig them out of the hole they have swords at the ready invade they need to attack.
Early the next day Rilian goes to meet his father as his boat returns to Narnia. They hug and the old man passes away. Aslan comes to take them back to their earthly world but as they journey with him they come to a stream where the old king is lying in death. As the water flows over him his youth is restored. He has always wanted to see the earthly world so Aslan says he can join them for their return. They arrive back in the school garden exactly as they had left it. When the. Lilies see the enormous lion they are terrified and they are also avenged when the children and a newly-young Caspian hit at them with their swords. The school changes for the better and becomes almost enjoyable, and Eustace and Jill remain friends always.