From the back cover:
Lonely, shy Irena sees Bella, a beautiful toy hourse, in a junk store window on Christmas Eve. She begs her parents to let her have the toy horse as her Christmas present. She takes it home and cares for it by brushing it, bathing it, and loving it. She even gives it a warm blanket and some hay. One night Irena hears a neighing outside her window. Her toy horse is alive! Barefoot and dressed in only her nightgown, Irina runs out to Bella, and they ride joyfully through the night. Irina and her enchanted horse share night rides throughout the winter.
But in the spring Bella leaves, to run with the wild horses. Irena is heartbroken, forgetting the storekeeper's warning: Never say that any creature on this earth is yours or you'll shed bitter tears. Next year when the wild horses pass through, Bella brings Irina an injured foal who cannot keep up with the herd. Bella doesn't stay, but she leaves Irina another horse to love and cherish.