Heart takes in an abused, pregnant mare against the wishes of her guardian, Simon Pratt, and after the mare's weak and disfigured foal is born, Heart must find a way to protect them when Simon decides to sell them for slaughter.

"Heart must hide her beloved unicorn, Moonsilver, from Lord Dunraven's hunters, so she and her animal friends join a troupe of traveling minstrels. Everyone thinks Moonsilver's horn is fake--just part of the act. Then Moonsilver heals a badly injured boy and the unicorn's secret is in danger. Can Heart's mysterious silver bracelet help her find the one place Moonsilver might be safe?"--p. [4] of cover.

Heart hopes to evade Dunraven's men to order to reach the Sunset Gates, which seem to be the key to finding her family and saving the unicorns.

In order to find a safe haven for her unicorns, Hart must solve the riddle of Castle Avamir.