Megs is a student at Oxford University in 1950 whose eight year old brother is so ill that he is unlikely to live another year. While Megs loves equations, her brother George loves the new book "The Lion, the Witch, and the Wardrobe" by C.S. Lewis and he convinces his sister to meet the author to learn about the book's origins. Through those conversations, she comes to better appreciate fiction.
The little that is said of mathematics in the book makes me think the author does not really know much about it. When she first meets Lewis, Megs says "I attend Somerville College reading maths". However, besides vague references to "figures" and "sums", the only math that is ever mentioned are Einstein and Newton's physical theories. None of that is what a student of mathematics was likely to be learning at Oxford in the midtwentieth century.
If I was to quibble about this mathematical detail, however, I would be missing the point (and probably confirming the author's prejudices about mathematicians). The point, which the book bluntly states in this conversation between Megs and her friend Padraig, concerns the importance of stories:
(quoted from Once Upon a Wardrobe)
"Surely you know physics is more important than fiction?" This is an absurd discussion. I can rightly enjoy a good story, but thinking novels are the same as Einstein and Newton's theories is absurd.
"I think they are neither more important nor less important," he says to my surprise. "No. Not one bit."
"But we cannot understand our world without the genius of the mathematicians. It's a language of the universe," I say.
"You are ignoring imagination; you need it for your work too.
But I can't really understand my life without stories. They offer me... they offer all of us the truth in their myths mysteries, and archetypes."
[...]
"I'm sorry. I'm being a dolt. I just can't figure out how to help George, and this whole thing is frustrating me. At least with a math problem I can work on it until the right answer shows up."
"Shows up?" Padraig sits again also, then scoots even closer so our knees are touching. "Like a character."
[...] I think what I was trying to say is that when my fictional characters show up, or the ones you're reading about in that book, they have a place they're going. A journey. A math problem does too. I've seen Father spend years on one equation until it shows him the way it is meant to go. That's what a story does with me. I'm not trying to convince you, Megs with the flashing blue eyes, that my work is more important than yours, but maybe it's just as important."

I certainly would not argue with the idea that fiction is important and has a role to play in our understanding of the world. (That, in part, is why I maintain this website about mathematical fiction.) However, at the risk of again being a stereotypical mathematician, let me also say that the book goes a bit too far for my tastes when it suggests that mathematics is nothing more than another kind of story telling. Similarly, although I agree that mathematics also requires imagination (a true statement that the book supports with a quote from Albert Einstein), that doesn't mean that mathematicians are making use of stories when they utilize it. Fiction and mathematics are just two of many human endeavors which require imagination.
Callahan, who also wrote "Becoming Mrs. Lewis", clearly has an affinity for and much knowledge about the author C.S. Lewis. And, if there really is anyone who fails to see that fiction is more than just a form of entertainment, then reading this bittersweet tale could help them see the error of their ways. However, I did not particularly enjoy it or get much from it. (The only thing I can really say I learned from it, assuming it is true, is the interesting fact that Florence Augusta Lewis, mother of the author of the Narnia books, had a degree in mathematics. I have not been able to confirm this, but I have no particular reason to doubt that she did. If so, it is indeed a very rare achievement for a British woman of that time!) 