The Sorcerer in the North (Ranger's Apprentice, #5)

Time has passed since the apprentice and his master, Will and Halt, led the Araluens to victory against invaders, and Will is now a full-fledged Ranger with his own fief to look after. The fief seems sleepy -- boring, even -- until the Skandians show up and Will has to deal with the threat. Shortly thereafter, Will goes on a secret mission. Joined by his friend Alyss, Will is thrown headfirst into an extraordinary adventure propelled by fears of sorcery, and must determine who is trustworthy to the king and who is trying to take his throne.

Will and Alyss must battle growing hysteria, traitors, and most of all, time. The king is fading, but when Alyss is taken hostage, Will is forced to make a desperate choice between loyalty to his mission and loyalty to his friend.