In the movie, the Pevensie children are staying with their annoying cousin Eustace when they are suddenly transported to Narnia through a painting hanging on the wall. They find themselves aboard the Dawn Treader, a magical ship captained by Caspian, who is now King of Narnia.
Their quest is to find the seven lost lords of Narnia, who were banished during the reign of Caspian's evil uncle, Miraz. Along the way, they encounter various obstacles including sea monsters, slave traders, and a fierce dragon.
Eustace begins as a selfish and obnoxious character, but through his adventures with the others, he learns the value of teamwork and loyalty. He even transforms into a dragon himself, but eventually reverts to his human form.
The group eventually reaches the edge of the world, where they meet the great lion Aslan, who tells them that their adventures have been a test of their character and faith. They return to England, with Eustace being a changed person and the Pevensie siblings realizing their time in Narnia has come to an end.
Overall, The Chronicles of Narnia: The Voyage of the Dawn Treader is a magical adventure story filled with action, danger, and important life lessons.