The Earth is under attack by an alien menace known as the Boazanian Forces.

Hrothgar is demonstrating his capabilities with his inventions and generous demeanor.

Hrothgar and slave rebels battle Boazanian soldiers; before they are captured, the rebels aid Hrothgar's escape with his airship; he pledges to return and rescue the rebels from the empire.

Mary Ann gives Hrothgar a new name, Ned Armstrong, and they fall in love.

Ned returns to Boazan to fulfill his promise and free the hornless slaves from Emperor Zambojil.

Dr. Smith introduces the five Volt machines to the cadets; Ned returns to Planet Boazan and the Boazan soldiers try to capture him.