Heart must solve the riddle of Castle Avamir, which is, among other things, "deep in a valley" and "over the moon, " in order to find a safe haven for the unicorns as the Gypsies leave Lord Levin's mountains for Lord Kaybale's plains.
With Moonsilver the unicorn disguised in armor, young Heart Avamir, searching for her Gypsy friends, attempts to travel unnoticed through the crowds gathered for the crowning of a new lord.
Heart sneaks back into her village when her beloved unicorn Moonsilver is injured by a hunter's arrow, seeking help from the healer Ruth Oakes, but she is forced to flee again when she learns the cruel Lord Dunraven knows about the unicorn's existence, with only the gift of Ruth's silver thread for help.
A lonely young girl named Heart finds companionship at last when she discovers a pregnant mare in the forest, but she has a fight on her hands when her mean-spirited guardian decides to have the mare and her disfigured foal slaughtered by the town soap maker.
