Alec and the Black Stallion go to the Irish coast after the horse suffers a racing injury, and while there, they encounter a shape-shifting kelpie that engages them in a race to save the life of a local girl.

While in Greece making a film about Alexander the Great, Alec Ramsay and the Black Stallion get lost and find an immortal city ruled by the tyrannical, ancient Thracian god-king Diomedes, whose flesh-eating mare, not yet tamed by Hercules, takes an dangerous interest in the Black Stallion.