Lucy and Edmund Pevensie, along with their cousin Eustace, return to Narnia and join Prince Caspian on an adventurous sea voyage aboard the royal ship, The Dawn Treader. As they sail across the seas, they encounter magical creatures like dragons, merfolk, and dwarves, as well as a group of lost warriors. Their journey leads them to the edge of the world, where they must face challenges and uncover new secrets to save Narnia.