But oh boy does it fill that hole well!
Plot: the Pevensie children are summoned back to Narnia to find that over 1000 years have passed, humans have taken over the country and the native Narnians have been all but wiped out. The boy-king Caspian, having been usurped by his uncle Miraz, sides with the few remaining Narnians and leads a rebellion against humankind. The Pevensies join in.
I read and reread these books when I was little and it always seemed to me that Prince Caspian was the weakest of the lot: it's basically a trudge through the woods followed by a battle followed by Aslan turning up and telling everyone where they got it wrong. Again. (Mind you, it's also the book that convinced me that those American fundamentalists that believe that C S Lewis is a dangerous heretic who will lead children to paganism in the guise of Christianity might have a point*.)
This movie fixes the weaknesses of the book beautifully. It makes the characters far more complex, rounded and believable than their author ever did. It gives Susan a proper role in this her final trip to Narnia. It soups up the antagonism between Peter (the old king) and Caspian (the king to be), and resolves it (in the necromancy scene) in a manner that I thought was brilliantly clever. It adds a whole new section where a commado raid is staged on Miraz's palace - it goes horribly wrong and a lot of Narnians die: interestingly the Narnians previously considered to belong to Evil Races (minotaurs, those broo things, wolves) are shown to be among the bravest and most self-sacrificing. Reepicheep the mouse is made to seem charming instead of being the irritating little turd he is in the books. And Aslan just about gets away without looking like a total tool.**
This is a dark film, full of violent (though bloodless) battle - even the mice cut throats, and a scene where Miraz punishes his soldiers is genuinely horrifying. The moral labelling of the characters is much less black-and-white than in the first film. The politicing among Miraz's courtiers is superb - I hooted with delight watching them manipulate him to his downfall. Trumpkin the dwarf, dry and sarcastic, is absolutely excellent: my favourite character. Caspian is cute if you like the pretty-boy thing. And the lead centaur is HOT.
But the best thing about this movie was how emotionally engaging it was: with the last of the Narnians falling victims of genocide, it conveys the despair and anger and desperation really well. You can totally understand why some of them want to call the White Witch up from the dead: I'd have been tempted myself.
* They (the fundamentalists) are still vile moronic blights on the face of humanity of course. They're just good at spotting heresy.
** I can't bloody stand Aslan. He throws people into horrible situations which he could have solved with the blink of an eye, they battle or muddle through as best they can, and then he inevitably turns up and tells them off for not doing as well as they should have. Not to mention him leaving innocent faithful Narnians to suffer and die for thousands of years and doing bugger-all to stop it. But we're getting into Apologetics grounds here so I'll stop ranting...