Karigan G'ladahon has fled from school following a fight which would surely lead to her expulsion. As she makes her way through the deep forest, a galloping horse pounds up to her, its rider impaled by two black-shafted arrows. With his dying breath, he tells her he is a Green Rider, one of the legendary magical messengers of the King, and makes Karigan swear to deliver their message he's carrying, giving her his green coat, with its symbolic broach of office. This promise given to a dying man changes Karigan's life forever. Pursued by unknown assassins, following a path only her horse seems to know, she unwittingly finds herself in a world of deadly danger and complex magic, compelled by forces she cannot understand.