Will is small for his age, but agile and energetic. All his life, he has dreamed of becoming a great knight like the father he never knew, so he is devastated when he is rejected as an apprentice to Castle Redmont's Battleschool. Instead he is apprenticed to Halt, the mysterious Ranger whose uncanny ability to move unseen is thought to be the result of black magic.

Reluctantly, Will learns to use a Ranger’s secret weapons: a bow and arrow, a mottled cloak and a stubborn little pony. It may not be the sword and battlehorse he longs for, but when Will and Halt set out on a desperate mission to prevent the assassination of the King, Will finds that a Ranger’s weapons are not so useless after all...

Ranger's Apprentice 1: The Ruins Of Gorlan by John Flanagan