Set in the Wyoming territory of the 1880s, Geoffrey Scott starred as Marshal Jim Donner who discovered a secret kingdom of aliens from outer space (Chimerians) living beneath the town of Cheyenne.

Following a lead from a boy named Billy (Tiger Williams),Marshall Donner used a strange key he found and accessed an entrance to a mountain cave, but  was caught inside a elevator that transported him thousands of feet below the surface where he discovered a futuristic civilization. Billy ran off to tell the townsfolk about the marshal disappearing, but his reputation as a fibber worked against his efforts to rescue the marshal. 

Donner	Chimeria

Meanwhile, the marshal formed an alliance with a rebel faction of the aliens known as Partisans - led by Princess Maya (Pamela Brull) the daughter of the former ruler of the Chimeria - to overthrow the evil Emperor Thorval (Mark Lenard) and his spoiled, power-hungry daughter, Princess Tara (Diane Markoff /Stepfanie Kramer).

Thorval controlled his people with the "Compliatron," a mind controlling device powered by gold that enslaved his citizenry and would do the same to the surface dwellers when Thorval's plan of conquering the  world above his domain was realized.

Living on the surface would have its problems because Thorval's people had to wear special breathing devices to cope with the upper atmosphere and hooded clothing to protect them from the sun.

Outlaws known as the Phantom Riders supplied Thorval with his precious metal needs by stealing shipments from the Aurora Mining Company.

"What manner of demon threatens to tear the marshal limb from limb? Is Billy's story of a secret mountainside cavern ever to be believed? Will Maya be able to save her people from Thorval's evil legions? Don't miss the next spellbinding chapter of...THE SECRET EMPIRE."

The serial ended with the defeat of Thorval and the destruction of the Compliatron. Thorval's' brother Demeter, a Partisan, allowed his treacherous brother and Princess Tara to escape to the stars while he stayed with his fellow aliens and lived in peace beneath the surface.

The orphan boy Billy, who had subsequently been kidnapped by Thorval  stayed with the aliens. Marshal Donner and his friend Milly Thomas (Carlene Watkins), who joined in the fray to defeat Thorval, returned to the surface.

TRIVIA NOTE: The serial was based on the matinee serial The Phantom Empire (1935) starring Gene Autry as a singer who came across a forgotten underground civilization of Muranians with advanced technology that threatened the surface world.

On the TV serial, the surface world of Cheyenne, Wyoming was filmed in black and white while the subterranean world wase shot in color.

The program was one of three serials on the CLIFF HANGERS! series, including THE CURSE OF DRACULA and STOP SUSAN WILLIAMS.