While working in teams on a system of radio relay stations, the Robinsons’ work is disrupted by a powerful feedback signal that destroys the new stations. John deduces that the signal was part of an alien strategy to destroy their communications system prior to launching an invasion. A mysterious voice announces to the party that they are all now prisoners of the Kondor Nation.
When Will and the Robot find a cave full of electronic equipment they attempt to destroy it, but are stopped by Niolani, leader of the invasion. She reveals that she is the vanguard of a colonization party. She then goes to the Jupiter 2 and tells John and Maureen that she is holding their son captive and that they must surrender. To prepare for the arrival of the colonists, the men will work as slave labourers building a gate that will acclimatize the new arrivals to the planet’s atmosphere. On her planet, since women are regarded as superior, and the Robinson women are to be reeducated for their new roles.
Dr. Smith attempts to ingratiate himself with Niolani who is at first disdainful of his advances. Eventually, she takes him on as a sort of consort. Meanwhile, the others attempt an escape, but when it fails, they are threatened with death if they make any further attempt. Will and the Robot come up with a plan to destroy the archway they are building before the new colonists arrive. They replace Dr. Smith’s aesthetic additions—sculpted putti—with statues moulded out of plastic explosive. Despite several setbacks, the Robinsons are able to detonate the explosives and destroy the gate just as the colonists are about to arrive.
Niolani is devastated by the failure of her mission and over her fate when she returns home in disgrace, having been bested by men. She will be condemned to menial work raising children. Maureen attempts to console her, telling her that child-raising is a rewarding task.
- The Purification Arch prop was also used as part of the Robot's internals in the next episode, Trip Through the Robot. It was also part of Sesmar's equipment in The Dream Monster.
- Where did Doctor Smith get all the art supplies?
- Niolani states that before any of her people can survive on the Robinson’s planet, they must first pass through a “purification arch” which will make it possible for them to breathe in the Robinson’s atmosphere. If this is true, how is Niolani herself able to breathe? She has not passed through any arch.
- The females of Niolani's race look human, while the males are plain and gray colored.
- When the arch blows apart it comes near to falling on Francine York ,who played Niolani.
- The alien rocket ship is the same rocket engine seen in several episodes. This time, the ship is seen landing and filmed in reverse, as it leaves, after the arches destruction.
- At the end of the espisode, Penny seems to have forgotten her superiour atttitude that she had assumed, thinking women are superior to men.