Into the Wardrobe

What was the Forgotten Story that Lucy forgot in The Voyage of the Dawn Treader?

I think that this is really a story within the story.. its another allegory to Christ, at least the Jesus of Nazareth we would call Christ, as Aslan is the Jesus figure in Narnia. This really stems from all the things Lucy remembers and of what we see in the last four days of Jesus in jerusalem: the cup at passover which becomes the symbol of the new covenant; Peter's use of the sword in Gethsemane, and Christ's admonition that violent men take the kingdom by force; the tree,, which is another word for the cross ("cursed is any man who is hung from a tree), and the green hill, which is the common site of the resurrection, especially in English paintings.
Lewis tells us in several books and writings that all good myths and stories which really speak to us of longing and satisfaction stem off of the one story of Christ. And the fact that Aslan will continue to tell her the story, really enforces Lewis's notion of God calling out to humanity and reminding us everywhere we look in nature and in ourselves of the story of Christ.