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.

Twilight is Lauren's pony, and they share a secret. When Lauren says the magic words, Twilght turns into a beautiful unicorn. Lauren and Twilight want to help Shadow, a young pony who is afraid to jump. Twilight uses his unicorn magic to help Shadow overcome his fears. But then one scary, stormy night, Shadow is in danger and he must find the courage all on his own.

"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.

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