Peter, Susan, Edmund, and Lucy Pevensie sit at a railway station waiting for the train that will whisk them away to boarding school. For those unfamiliar with the concept, a boarding school is what you'd get if a regular school and a jail had a baby (read: you live there, all day, all school year). Luckily for the Pevensie children, a magical force does the whisking instead of the train, and they find themselves in a thick thicket.
As any curious children would, they go exploring. They discover they are on a forested island with a bunch of ruins. Peter figures out that they've returned to Narnia, and that these ruins are their old castle of Cair Paravel from back when they were kings and queens. To prove it, they find their old treasure room where their awesome king and queen swag remains. Nice.
The next morning, they rescue a dwarf from being drowned by a couple of soldiers. NBD. The dwarf thanks them and tells them he fights for King Caspian, a Telmarine and the true king of Old and New Narnians alike. Old Narnians? New? Caspian? Sounds like some backstory is in order, and the dwarf kindly obliges.
Caspian was the son of the king, but after the king's death, his uncle, Miraz, ruled as a regent since Caspian was too young. Caspian learned of Old Narnia and the ways of Aslan first from his nurse and then from his tutor Doctor Cornelius. Cornelius taught the young king until Miraz's wife became pregnant. Realizing Caspian's life was in danger, Cornelius snuck him out of the castle and sent him riding toward the southern woods.
There, he met the Old Narnians, including Trufflehunter the badger; Trumpkin and Nikabrik the dwarfs; a couple talking bears and squirrels; and some centaurs and fauns for good measure. What self-respecting magical wood doesn't have centaurs and fauns bound about, right? With the rightful king on their side, the Old Narnians decide the time for a civil war has come.
Seems like a good idea—until they start to lose. In a moment of desperation, Caspian blows Susan's horn, given to him by Dr. Cornelius, and Trumpkin heads to Cair Paravel to see if Aslan or some other type of divine assistance has shown up to help (30 minutes or less or the miracle is free!).
And this is where we came in to the story. Trumpkin and the Pevensie children agree to beat feet and get to Caspian's aid as soon as possible. They get lost along the way and suffer hardships, including a non-talking bear, which apparently aren't as friendly as their chit-chatty brothers. When they get to a gorge, Lucy spots Aslan, but the others, save Edmund, don't believe her. They take the hard road and get even more lost.
When they sleep, Aslan comes to Lucy and tells her to wake the others. Although always a difficult task to wake up campers, she manages, and they all follow Aslan to Caspian's camp. There, they find Nikabrik trying to resurrect the White Witch, source of evil, eternal winter, and enemy of Christmas. Since Peter and Edmund are fans of good things and Christmas presents, they stop Nikabrik and his corrupt companions.
With his new army outnumbered, Peter challenges Miraz to single combat, which Miraz accepts because he feels his pride is on the line. Peter and Miraz duke it out old-school style with swords and armor. But their duel is cut short when two Telmarine lords, Glozelle and Sopespian, betray and stab Miraz in the back (literally). The Telmarines fight the Narnians, but with Peter and Edmund in command, the Narnians beat them back to the river.
Wait, but what were Susan and Lucy up to during all this? Well, they were with Aslan invading the town of Beruna. With the gods and creatures of old, they scared off the Telmarines who were scared of the old ways while accepting others into their fold.
Victory and success! Caspian becomes King of Narnia, and the Old Narnians can again live freely in their land. Aslan allows the Telmarines who wish to remain in Narnia to do so, but teleports the others to an island in our world.
As for Peter, Susan, Edmund, and Lucy, they have to return to their own world, too. For Peter and Susan, it's bittersweet as Aslan says they've become too old to return to Narnia again (a.k.a. too tall to ride this ride). It's hinted that Edmund and Lucy will return one day, though. And with that, they return to the railway station and the promise of the coming school year after one last summer adventure.