Jonathan Dark (Jandar), earthman mysteriously transported to the Jovian moon of Callisto (or Thanator), has in concert with the native Ku Thad succeeded in freeing the city of Shondakar from the occupying Black Legion and the opportunistic Zanadarian Sky Pirates.

Plopped into the Corund Laj, Thanator's greater sea, Jandar finds himself close enough to land to swim to safety, only to be enslaved by the mercantile Perushtar who rule its waves.

He has his hands full simultaneously surviving as a gladiator, hiding his identity as the Sky Pirates' arch enemy, and stirring up a revolt among his fellow slaves.

Zanadar is destroyed and the menace of the Sky Pirates ended by the explosion of the pocket of "lifting gas" over which the city is built and on which its air power is based.

Reviewing this book along with the preceding two volumes, Lester del Rey found the series to be "fairly entertaining reading," but noted that Carter had "copied every trick of Burroughs, including those that are faults.