When two boys from a remote island off the western Irish coast venture to the forbidden Island of Horses, they find a mysterious tame black colt and unexpected danger.

We go to the island of horses -- We hear the wild horses and catch some eels -- We find the secret valley -- We bring home the colt -- We hear from Mike Coffey -- We hear the grandmother's story and sail to Rossmore -- At Stephen Costelloe's house -- We set sail in a Nobbie -- Shipwrecked -- We come ashore and meet Luke the cats -- We make a valuable friend -- The grandmother goes sailing again -- Back to the island of horses -- Mike Coffey meets his match, and the grandmother receives a visitor -- The end of the story.