Flame of Iridar and Peril of the Starmen

"From out of the gloom in the dungeon where Chandar, Prince of Orin, awaited a tortured death, Sarkond the Enchanter shimmered into reality. His emerald eyes gleamed with an inhuman lustre as, with a soundless command, and without a touch, he mysteriously released the chains which bound Chandar. The prize Sarkond then offered Chandar was freedom, the price-
the timeless struggle between two opposing forces... a man who used the secret powers of the universe for his own schemes... a man who vowed to stop him by his own strength and wits. Five inhabited planets had been shattered to nothingness by the Oligarchs of Brionmar. The sixth was to be Earth...
The people of Earth welcomed the smiling and friendly starmen of Brionmar with gracious approval. The starmen asked only one favor- "Let us make a geological survey of your planet's surface.""
Who would suspect the true purpose of the survey-Who would stop the countdown to the destruction of Earth which began the moment the starmen landed?"