Originally published as the fifth Narnia novel, The Horse and His Boy is now third after a resequencing of the books put them all in a rough narrative chronological order. This story takes place during the years that the Pevensie siblings — Peter, Susan, Edmund and Lucy — ruled Narnia (growing into adulthood doing so) before returning home through the wardrobe (as children again), and it opens not in Narnia but Calormen, an arid land far to the south much inspired by The Thousand and One Nights. In Calormen, slavery thrives, and young Shasta finds he has the chance to escape such a fate when he meets Bree, one of the talking horses of Narnia, captured and forced to serve a haughty nobleman. The two ride off in the dead of night together, and soon meet two other fugitives, the girl Aravis and the mare Hwin. Aravis is fleeing the dire prospect of marriage to the vizier.
The group hopes to make their way north to the fabled land of Narnia, where both horses are originally from, and where Bree (a little smarter than Shasta and not a little stuck-up about it) suspects Shasta may be from as well, due to his fair complexion. Shasta knows he is an orphan, and perhaps there is now a chance to find out where he really came from.
Trouble arises when they are forced to pass through the opulent but congested Calormene capital of Tashbaan, and are separated. Shasta is mistaken for a prince by a visiting retinue from Narnia, consisting of Queen Susan and King Edmund. The mistake is quickly sorted out when the real prince (who bears an uncanny resemblance...hmmm...) returns from playing hooky, but before this happens Shasta overhears some royal business: that Susan, once betrothed to the prince Rabadash, has called off the engagement on account of what a creep he is. But the Narnians fear that Calormen's ruler, the Tisroc, will imprison Susan once he hears the news, and so they quickly make plans to escape the city by stealth.
Aravis finds herself in a different pickle. She is recognized by one of her shallow girlfriends, who has married well (leading to some delightfully funny interaction). After Aravis convinces her friend none too easily that she must escape the city and her father's marriage plans for her, the girls unwittingly end up trapped behind a sofa in the Tisroc's own chambers, where they overhear private plans by Rabadash to invade Narnia in retaliation for Susan's flight. Soon, boy, girl, and both horses manage to reunite at an appointed meeting place. And the race is on to cross the vast desert separating Calormen from Narnia in advance of Rabadash's army, and get the warning out.
Whereas both The Magician's Nephew and The Lion, the Witch and the Wardrobe were children's fables with a fairly simple narrative structure, The Horse and His Boy is a full-blown adventure novel, aimed squarely at readers who were children when Wardrobe was first published, but teens with more sophisticated tastes now. For the first time, Narnia and its environs feel like a real world, not just a fabulous backyard paradise created in an instant by an act of Aslan's will. The scenes in Tashbaan are, in fact, surprisingly evocative of place, and in their vividness one can clearly see the fondness Lewis must have had for the Arabian Nights legends that helped to inspire them. This novel is also far less of an overt Christian allegory than Wardrobe or The Last Battle (Aslan makes his entrance near the end, doing his godly bit to guide our heroes when they need that extra helpful nudge), though one wonders, in the end, if the extremely patronizing and clownish depictions of the very Arab Calormenians has some roots in age-old Christian/Islamic animosities. It isn't as grotesquely racist here as it is in The Last Battle. But the point is made that Calormen has a cruel and ignoble culture compared to those of the north.
Theopolitics aside, The Horse and His Boy is a smashing adventure, and one of the series' more mature volumes. For my part, the story does lose a bit of steam at the climax, when the freshness of the exotic southern setting gives way to a pretty traditional and familiar-looking have-fun-stormin'-the-castle battle scene, and we find out something screamingly obvious about Shasta's true identity. But I found it immensely enjoyable all the same, and the one book of the series that shows the breadth of C. S. Lewis's creative vision.