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 laborers building a gate that will acclimatize the new arrivals to the planet’s atmosphere. On her planet, since women are regarded as superior, 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 her 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 take advantage of this distraction to escape the force field themselves, and 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 molded 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, and that equality between the sexes is a good thing.
- The Robot always knows exactly what is going on, and yet they never consult him or listen to his warnings.
- 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 arch is decorated with Christmas lights.
- It's really good that the box containing the plastic explosives is labeled "Plastic Explosives" in red letters; otherwise we wouldn't know what was in it.
- Will sure takes his time putting the plastic explosives on the arch, yet no guards walk by.
- The females of Niolani's race look human, while the males are plain and gray colored.
- Niolani is dressed like an alien; her consort Dr. Smith is dressed like an ancient Roman soldier?
- Why do John, Don, and Will need to be stationed at the arch when the alien colonists arrive? Their work building the arch is finished.
- The plunger for igniting the plastic explosives is large, bright red, and in plain site the entire time, and yet no one notices it.
- Will yells "Now!", John socks the male guard, the Robinsons all scatter, and yet Niolani shows no signs of suspecting foul play.
- 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 arch's destruction.
- At the end of the episode, Penny seems to have forgotten her superior attitude that she had assumed, thinking women are superior to men.
- The theme or object lesson in this episode seems to be that women are equal to men, and also that patriarchal attitudes toward women are wrong. At the beginning of the episode Will jokingly makes disparaging remarks about "girls." Then Niolani arrives who is from a matriarchal culture in which the tables are turned: women control everything and disparage men, who are treated as stupid and weak and only occupy a servile role. The episode seems to be trying to illustrate the unfairness of patriarchy by turning the tables and portraying a society in which women have all the power and men are second-class creatures. To underscore the point, at the end of the episode June speaks in favor of gender equality.