In chapter 8 of The Horse and His Boy, Prince Rabadash outlines a plan with two objectives: to capture Queen Susan and, eventually, to conquer Narnia.

Rabadash's plan is to ride across the desert to Archenland with a small company of horsemen. From Anvard, the capital of Archenland, he will proceed through Narnia to Cair Paravel, arriving there before the Splendour Hyaline, which is carrying Queen Susan home by sea. When the ship arrives at Cair Paravel, he will kidnap Susan and ride back to Anvard.

Once Rabadash has established a base in Anvard, the Tisroc will be able to build up his forces there until there is a large enough army to attack Narnia, something he could not do immediately, as a large army would not be able to cross the desert. This plan is attractive to the Tisroc, who does not care much about his son recovering Queen Susan, because it allows him to attack Narnia without declaring war. He will not formally endorse Rabadash's expedition and will appear to be unaware of it.