The producers and writers behind "The Chronicles of Narnia: Prince Caspian" say the same thing when describing the challenge they faced bringing the novel to the screen.

The problem, all agree, is that the second book in the classic seven-book fantasy series by C.S. Lewis is not structured like a movie.

The book's plot looks like this: The royal children from "The Lion, the Witch and the Wardrobe" are whisked back to the magical land of Narnia, where they meet a grumpy dwarf, who tells them a long, sad story that doesn't involve them about a prince they've never heard of named Caspian. So Peter, Susan, Edmund and Lucy Pevensie decide to help, which leads to a long, long walk in the woods that eventually brings them to Caspian. Then there is a battle. The End.

That doesn't exactly scream, "Summer movie!"

"Through the magic of C.S. Lewis, that all works quite well on the printed page," said co-producer Douglas Gresham. "However, it's almost impossible to make that plot work on the screen. ... In terms of its story and message, I would say that 'Prince Caspian' is impoverished, in comparison with 'The Lion, the Witch and the Wardrobe.' But while it may be poorer, as a story, I believe we have been able to make it into a better movie."

To do that, the team assembled by Disney and Walden Media decided to radically restructure the plot, including adding a second act that is not in the novel. That is sure to cause concern among legions of Lewis loyalists, which is a large crowd since sales of "The Chronicles of Narnia" have topped 100 million. The movie version of "The Lion, the Witch and the Wardrobe" grossed $748 million at the global box office.

In this case, it truly helps to know that Gresham -- in addition to being a producer -- is also Lewis' stepson and has been on a 30-year quest to turn the Narnia novels into full-scale motion pictures. Needless to say, he has played a strategic role in talks about artistic changes in Narnia.

"It would be hard to find someone who knows these stories better than I do or cares more about them," said Gresham, whose mother, poet Joy Davidman Gresham, met and married C.S. "Jack" Lewis during the years when the Chronicles were published. "The Narnia stories are a big part of Jack's legacy and, believe me, I am aware of that."

This has been a joy and a burden. Parts of "Prince Caspian" were filmed in the Czech Republic and, while in Prague, Gresham was introduced to the American ambassador. He wryly notes that, when the ambassador inquired about Gresham's role in the project, producer Mark Johnson had a simple reply: "Oh, he's to blame."

The key, said Johnson, is whether the messages in these books remain intact.

"The themes are the most important things," he said, during press events in New York. "You have to say, 'What is this movie about?' The first one was about a certain kind of faith and this one is about losing faith and then regaining it."

On one level, explained Gresham, "Prince Caspian" remains an adventure story about how the kings and queens from Narnia's golden age return to a troubled land and fight to restore "truth, justice, honor, glory and a sense of personal commitment and responsibility" during troubled times. However, the Pevensie children also struggle to believe that Aslan -- the Christ figure in Narnia -- will return and guide them.

The High King Peter, in particular, struggles with the "sin of pride" and his desire to prove he is still in command, said Gresham. This leads to a new twist in the plot, linked to an assault on the castle of the evil King Miraz.

"If anything, this theme that Peter has to regain his faith in Aslan is stronger in the movie than it was in the book," he said. To state this in terms that Narnia lovers will understand, if the younger King Edmund had to face his sins in "The Lion, the Witch and the Wardrobe," then Peter faces a similar crisis in the new movie.

"This is something we all have to deal with in life," said Gresham. "We all have to realize that no matter how far we stray, there's only one way to come back."