The second installment in The Chronicles of Narnia and the fourth book chronologically. A year after the events of The Lion, the Witch, and the Wardrobe, the four Pevensie siblings are summoned back to Narnia, where a thousand years have passed from the perspective of that world's relative time flow. Narnia has mostly been overrun by the descendants of an exiled group of pirates from Earth called the Telmarines (now ruled by the usurper Miraz), but the rightful heir, his nephew Caspian, has magically summoned Narnian help, receiving it in the form of the Pevensies. Once the Telmarines have been beaten back and Caspian proclaimed king, Peter and Susan are told they are now too old ever to return to Narnia.
This book provides examples of:
Agony of the Feet: Reepicheep and his people inflict this on the Calormene soldiers during battle. Justified in that, being mice, they cannot reach higher than the belt.
Arbitrary Skepticism : In Prince Caspian, Edmund is the only one to remember that Lucy was right about Narnia in the previous book, while Peter and Susan - who ought to know better - are reluctant to believe her when she says she's seen Aslan.
Artistic License - Biology: Reepicheep the talking mouse has lost his tail in battle, and he argues with Aslan over whether it needs to be regrown. Both of them seem to think a mouse's tail has no practical value, and is of use only as a badge of honor or vanity, but the tails of mice and rats are actually important thermoregulatory structures, without which he'd be quite vulnerable to heat stroke.
“I am hunger. I'm thirst. Where I bite, I hold till I die, and even after death they must cut out my mouthful from my enemy's body and bury it with me. I can fast a hundred years and not die. I can lie a hundred nights on the ice and not freeze. I can drink a river of blood and not burst. Show me your enemies.”
Cool Gate: "The Door in the Air" — it leads wherever Aslan wants it to lead.
Decapitated Army: Subverted. Miraz's death doesn't lead to Telmarine surrender - in fact, the lords doctored his murder to look like he's been traitorously killed by Narnians and motivate the Telmarine army to fight.
Deus ex Machina: Aslan comes in during the last battle to help the Narnians win after they began to lose hope.
Does Not Like Shoes: After arriving in Narnia, Lucy and Edmund, despite being told it will be cold come the night and not knowing what they could come across, decide to explore barefoot for no reason at all.
Narnia Time: The book lays out the remaining rules for this. While The Lion, the Witch, and the Wardrobe established the "time does not pass on Earth while you are in Narnia" rule, this is the book that codifies "while you are not in Narnia, any amount of time may pass in Narnia."
Sequel Hook: There is brief mention in the novel of seven lords who were loyal to Caspian IX who were unafraid of the sea. Searching for these lords is the impetus for The Voyage of the Dawn Treader.
Sibling Rivalry: Specifically between Susan and Lucy. Susan believes Aslan favors Lucy; Peter takes Susan's side in an important argument because she's older and talking logically, while Lucy's suggestion makes no (visible) sense. The film plays it up even more.
Spare To The Throne: In the backstory, Miraz, who killed Caspian's father to take the throne. In a way, he kept Caspian alive afterwards to function as this - if Miraz would die, at least the crown would stay in the family. The vent that kicks off the main plot is that Miraz has a son, who would become this trope to Caspian... unless Caspian were to die first.
In the film, Lord Sopesian and Lord Glozelle seek to overthrow King Miraz by provoking him into accepting Peter's challenge of a mano-a-mano sword fight in hopes that he'll be killed. When this doesn't happen, Sopesian stabs Miraz in the back. Glozelle gets a Heel-Face Turn, though.
In the book, Glozelle is the one who stabs Miraz to death, as revenge for his ex-leader insulting him before the duel with Peter takes place. Both him and Sopesian end up killed in battle.
Tearjerker: The Pevensies' sad realization that all their friends from the first book/movie are long gone.
On that note, the ending of the castle fight scene. A bunch of Narnians are trapped in the castle, and as Peter looks back, he realizes one of Glenstorm's sons are trapped in the castle. The look on his face when he turns around is heartbreaking.
"The Reason You Suck" Speech: Trumpkin gets a short but effective one in the film; when he's brought before Miraz and Miraz hits him across the face for pretty much no reason, he gives the king a Death Glare and simply says "And you wonder why we don't like you?"
Too Awesome to Use: Caspian is reluctant to use Susan's horn since there might be an even greater need for it in the future. Nikabrik points out that, by that argument, he will never use it until it is too late.
Although see Decapitated Army above: There's no guarantee that assassinating Miraz will see everyone instantly accept Caspian and his Narnian allies as the new regime, especially with a couple of Dragonswaiting to take his place. (It should be added all this only applies to the film: The book and BBC adaptation have a much simpler failed ambush where Caspian never has the opportunity to kill Miraz.)
With Due Respect: Inverted: Trumpkin argues strongly against Caspian's plan and then volunteers to do the job he just argued against. He explains to the prince that having given advice, now it's time for him to take orders.