The Voyage of the Dawn Treader
The two youngest of the four children - Edmund and Lucy - are staying with a nasty young relative, Eustace Scrubb over the holidays. The three of them are drawn into Narnia together through a painting of a ship at sea and find themselves on a real ship in Narnia with their old friend Caspian from the previous story. He is in the midst of a great sea voyage through many lands beyond Narnia, searching for great men of Narnia the previous king had sent into exile.