Eustace has to return to school after his adventures in Narnia with his cousins Lucy and Edmund. He explains to his cousins that Experiment House is less of a school and more of a place where they experiment with the children there. Upon his return, the bullies find that he is no longer sucking up to them, but standing up to them. He eventually has to run away from them. He finds Jill Pole, who is not too pleased by his company at first. After he convinces her that he has changed, Eustace tells Jill about his time in Narnia. They start asking Aslan for help out of their situation as the bullies continue to approach them. They decide to try a door that is usually locked and find it open. They make it through just in time to find themselves in a different world.
Jill is fascinated by the cliff that they stand on. She walks over to the edge and continues to look down, even after Eustace asks her to back away from the edge. Eventually, she does start to fall, but Eustace saves her, plunging down the cliff himself in the process. While Jill is distressing about losing her friend, Aslan appears to her. She admits that she was showing off and Aslan tells her that he blew Eustace to Narnia, and she will join him shortly. Aslan gives Jill signs that she must follow to find the lost Prince Rillian, King Caspian's son. She floats down to Narnia and tries to tell Eustace that he must great an old, dear friend, but he doesn't recognize anyone. A ship sets sail and Eustace learns that his friend, King Caspian, was on that ship.
Two owls come to Eustace and Jill in the night and lead them away to the Parliament of Owls. The owls tell them that the Queen was killed by a serpent and that Prince Rillian went searching for it every day to avenge her. They believe the Prince to be under a spell of a witch. The owls then lead them to a Marsh-wiggle named Puddleglum who will be able to help them on their journey.
They start traveling to the north to reach the Ancient Ruined City of the Giants. However, they are detoured from their Journey when they meet the Lady of the Green Kirtle and her speechless night in black armor. She tells them to visit the giants for refreshing before continuing on their quest. Against Puddleglum's suggestion, the two tired kids agree. Aslan appears to Jill one night and berates her for forgetting the sings and going astray. He points out in the distance the worlds "Under Me," and Jill realizes that is where she must go. Eustace and Jill find that the giants are planning to eat them and make their escape under the Ruins into Underworld.
They are taken to the queens chamber to find what should be done with them. They meet the man that is was in black armor on the road with the Lady of the Green Kirtle. He advises them to stay away from him while he is in his hour of madness. They leave him so that he can be bound, promising that they will not release him, no matter what he says. They return to find the man tied to a chair. He begs them to release him to break the spell, in the name of Aslan. They debate the choice for a while before agreeing to release him because of the signs. The man is freed of the spell and introduces himself as Prince Rillian. The witch returns and tries to wipe their memories away, but fails when Puddleglum stomps out her enchanted fire. the Lady of Green Kirtle then becomes the serpent to kill them all. Rillian beheads her and Underworld starts to fall apart. The gnomes are freed of the spell as well and return to Bism, their land further down in the earth.
They follow the underground tunnels until they see light. Puddleglum lifts Jill up so that she can see through the whole and they are rescued by fawns. Prince Rillian is returned to Cair Paravel just in time to witness King Caspian's death. Eustace and Jill are transported to Aslan's land, where they see Caspian in the river. Aslan makes him young again, but not to return to Narnia, where he'd be a ghost. Eustace and Jill are returned to their own world where Aslan and Caspian help to scare off the bullies.
The Chronicles of Narnia:
The Lion, The Witch, and the Wardrobe
The Voyage of the Dawn Treader
The Silver Chair
The Horse and His Boy
The Magician's Nephew
The Last Battle