Edmund Pevensie, played by Skandar Keynes, is on the brink of adulthood yet finds himself grappling with feelings of envy towards older men, prompting him to attempt enlistment in His Majesty's Army. However, his scheme fails spectacularly when his sister, Lucy, portrayed by Georgie Henley, reminds him of the grocery shopping he was supposed to assist her with. Meanwhile, Lucy is preoccupied with envy towards any girl she deems prettier than herself, often comparing herself to her older sister, Susan, who is currently in America.
Due to the ongoing war, the Pevensies are evacuated once again. This time, while Peter and Susan are sent all the way to America, Edmund and Lucy are diverted to their insufferable cousin, Eustace Clarence Scrubb, enacted by Will Poulter. Eustace lacks any semblance of social etiquette and treats his cousins with the same disdain he reserves for his vast collection of insects.
One day, while sharing a room, Edmund and Lucy discover a painting of a magnificent ship on a vast ocean, reminiscent of their past adventures in Narnia. As they admire it, the scene begins to shift, and water starts to seep into the room. Eustace, initially scoffing at their imagination, unwittingly exacerbates the situation when he tears the painting from the wall, causing the room to flood. Before long, they find themselves swimming in a real ocean and are rescued by the ship, which turns out to be Narnian!
They are greeted by none other than King Caspian, portrayed by Ben Barnes, who has come to rescue the Seven Lost Lords of Narnia. This group of noblemen had vanished after being exiled by the treacherous Lord Protector Miraz, and Caspian's quest is to locate them. Joined by a talking mouse named Reepicheep and others aboard the vessel known as the Dawn Treader, the adventure to save Narnia unfolds.
As the journey progresses, the crew encounters numerous challenges, including slavers in Narrowhaven, revealing Edmund’s bravery and Lucy's resourcefulness. They soon realize that the Green Mist holds their past enemies and threats, as relics from their previous adventures resurface. Each character faces personal trials that test their resolve, from Lucy’s encounter with her own insecurities to Eustace’s haughty demeanor leading to his transformation into a dragon.
As their saga continues, the Dawn Treader journeys through various islands, combating mystical forces, and ultimately standing against the Dark Island where nightmares manifest into reality. The group discovers that to defeat their foes, they must lay down the Seven Swords of Aslan at Aslan's Table—a task that calls for unity and bravery from them all.
With myriad trials and encounters compelling them to confront their deepest fears and desires, the story culminates in an epic battle against the Sea Serpent, leading to Eustace’s redemption and transformation back into a boy. The crew's combined efforts bring forth light amidst the darkness, allowing them to reclaim the lost Lords and thwart the Green Mist's monstrous hold.
Aslan, the great lion and God-King of Narnia, guides them to the end of their journey, marking a poignant farewell as they realize their time in Narnia is coming to an end. Caspian chooses to continue his reign, while the Pevensie siblings must return home, forever changed by their experiences, leaving a legacy that hints at future adventures to come. The film closes with a longing farewell and a sense of nostalgia for the enchanting world and friendships they forged, exemplifying the enduring magic of Narnia.
