Tarzan wanders into a strange valley and encounters a bull ape in combat with a horde of tiny ten inch tall warriors on miniature antelope. He kills the ape and rescues a small warrior and makes friends with Prince Dorak and his warriors of Minian. But Tarzan’s arrival coincides with a major war campaign with the Tesmals, from another ant-men city. Tarzan joins them in battle and is overwhelmed by the vast force of numbers and taken prisoner. When he comes to he soon realizes that he has been reduced in size as he is led captive before the Tesmel king and the princess Zara. He is condemned to hard labor in the quarry with the other captured slaves. 

That evening he rescues a slave girl, Gazel, from the unwanted advances of a brawny slave. For fighting Tarzan is sentenced to a hundred lashes. He is befriended by other prisoners and learns that a scientist, Maro, is responsible for shrinking him. The next night after the beating and a day in the quarry Tarzan and Dorak overcome two guards, disguise themselves in their garb and rescue Gazel from Skar, the quarry master who has taken her for his own personal slave. After killing him in a sword duel Tarzan and his friends escape in an attic passage and exit in Princess Zara’s chamber. Before they can use her as a prisoner she triggers a trapdoor and the men fall into a pit with two civet cats that are full sized. Gazel and Zara then battle above and likewise fall into the pit. The two swordsmen kill the cats and find Maro in an adjoining cell, put there by the king who feared his downsizing experiments.

They break out and escape the city on fleet antelopes. Maro and Zara realize that they love each other and decide to return to the city and rule justly. Dorak departs to his own city. Tarzan learns from Maro that his diminutive size is only temporary and with that information heads into the jungle to battle his way back to his own African territory. In the ensuing weeks Tarzan battles or eludes many predators and after a sky battle with a hawk who has seized him, he regains consciousness on the ground aside the dead bird to discover that he has returned to his full normal size.

~ Adapted from Kenneth Webber's 2004 ERBapa article