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The Narnians were in Tashbaan because the Calormene Prince, Rabadash, was courting Susan for marriage. But the Narnians quickly realized that he wasn't the sort that Susan would wish to marry. So they planned a secret escape by boat at night. But after making their plans, Corin, the boy for whom Shasta had been mistaken, returns, so Shasta takes off. Meanwhile, Aravis has also encountered some trouble, as she is recognized on the street by one of her friends. So she hides in her curtained litter and goes back with her to the palace, along with the horses. She sends the horses, with a groom, out to the tombs outside of town, the arranged meeting place for the group, and schemes to go out herself through a gate in the palace gardens and cross the river. But as she sneaks down to escape, she sees the king coming, and hides behind a couch in one of the rooms. While there, she overhears that Prince Rabadash has discovered that the Narnians have made an escape, and that he plans to make a surpirse attack on Archenland, and then move on to attack Narnia. She then escapes, and carries this news to Shasta when they all meet back up at the tombs. They immediately set out across the desert toward Archenland. It is a tough trek, but they make it to a canyon that Shasta heard about from the Narnians. And following that canyon, they made it to the river, and the edge of Archenland. They could see Rabadash and his armies making their way across the desert, and searching for a ford of the river. So they hurried toward Anvard, the capitol of Archenland. But while they made their way through the mountains, they found a lion at their heels, and they rushed forward, taking refuge in the home of a magician. He urged Shasta to take the news to Anvard, so Shasta set out. He encountered the king, and passed along the news of the impending attack. They all set out for Anvard, to sound the alarm, but Shasta got separated in the fog, and found himself alone on the road. While wandering in the fog, he met Aslan, who told him that he had been the lion they had seen the two times before, guiding and hurrying them along so they could save Archenland. He also told Shasta that he had guided the boat to the shore when he was a kid, saving his life and puttin his life on its present course.
Shasta met up with some talking creatures from Narnia, as he found himself across the pass from Archenland in Narnia after his walk in the fog, and told them of the impending attack. They quickly got word to Queen Lucy, and Narnia gathered forces to help Archenland. Shasta met up with them as they headed back toward Anvard, and joined with them. When they arrived in Anvard, they stopped Rabadash's attack, and defeated his warriors. Shasta was met with a warm recetion by King Lune of Archenland, who told him that he was his long lost son, Cor. Corin was in fact his twin brother. And Cor was the elder, and thus the heir to the throne. They invited Aravis to come live in the palace as well, and some day she would marry Cor. The two horses made their way back to Narnia, to live happily as talking horses again.
This is another great story from Lewis. He builds great characters and an interesting story line. And again, he does it with some great depth. In The Horse and His Boy, he explores the theme of God's guidance, by looking at how Aslan guides the story, from helping the boat Shasta was afloat on to shore, to scaring Shasta and Bree into meeting up with Hwin and Aravis, to scaring and urging them on to hurry toward Archenland, to meeting up with Shasta and saving him from a treacherous fall as he walked over the pass in the dark. In various and unlikely ways, Aslan had been guiding the story the whole way. He never made up anyone's mind for them, nor did he overcome their wills to do it, but he still guided and directed events to their best outcome, though even in unexpected and sometimes painful ways. It is a great exploration of God's sovereignty and guidance.