In order to make important repairs, the Jupiter 2 sets down on an alien planet. As they do so, they mistakenly drag a nearby space capsule down to the planet with them. This act catches the attention of Zeta Galaxy Law Enforcement Bureau because the capsule contains something very dangerous.
Will notices that the Robot is very depressed and takes him exploring in the hopes of cheering him up. As they walk, they find the crashed capsule, but Will assumes it is just old space junk and goes away. A female voice speaks to the Robot from within the capsule and asks for his help. She pleads for the Robot to dig her free, and he does.
Two representatives from Zeta Law Enforcement Bureau approach John and Don to ask about the capsule, and John tells them honestly that he has not seen it. The officers do not believe him and they threaten the Robinsons, forcing the family to flee to safety inside the spaceship. The force field protects the Jupiter 2 from being damaged, but John begins to worry when the officers wait outside the ship all night long.
During the night, the Robot sneaks out of the ship and goes to a cave. Within the cave is a female robot, the fallen capsule’s passenger. She is evil, but the Robot is drawn to her all the same. She convinces him to reassemble her and to help her regain her strength.
When morning comes, the two officers attack the Jupiter 2 again and the Robinsons fight back. When the officers leave for a while, the Robinsons notice that the Robot is not on board the ship. Will is worried and goes to look for his friend. He finds the Robot in the cave with his female companion. The female robot wants to kill Will, but the Robot will not let her. The female robot then demands that the Robot get her an energizer, so the Robot goes to the Jupiter 2 to get one. The Robot tells John that he will trade Will’s freedom for the energizer, and John reluctantly gives it to him. Will is released and the Robot bids him good-bye, intending never to see the Robinsons again.
Back at the Jupiter 2, the Robinson family is very unhappy because their force field will not work without an energizer and they are now vulnerable to attack. The officers show up along with their leader, who tells the Robinsons about the contents of the missing capsule. He says the female robot in it is evil and dangerous, and that she once slaughtered an entire world. The Leader asks the Robinsons to help him find the evil robot before she kills again.
After hearing this, Will runs back to the cave to warn the Robot that his new flame is an evil murderer. The female robot says Will is a liar, and demands that the Robot prove his love and loyalty by killing the boy. Will steals the energizer and runs away. This angers the female robot, and she leaves the cave, determined to kill all the Robinsons.
John, Don and the space enforcers find the female robot just as she is about to kill Will. She steals back the energizer and is about to shoot the boy, but the Robot protects Will. The female robot is infuriated and kills the space enforcers. She says that she will come to the Jupiter 2 that night to kill the rest, and then retreats back to her cave.
The Robot rejoins his family at the Jupiter 2 as they plan their defence. John and Don construct a disintegrator with which they can destroy the female robot. When she arrives at their campsite, she urges the Robot to rejoin her and help her on her murderous quest of universal domination. The Robot refuses and uses the disintegrator to destroy her. The Robinsons are safe, but the Robot will suffer from a broken heart for a long time to come.
- The alien space station footage was originally from the 1958 science fiction movie War of the Satellites.
- In a minor continuity error, after the Robot throws a set of switches to deactivate the force field in front of the sleeping Dr. Smith, the camera then follows the Robot over to the hatch. The next shot shows Dr. Smith stirring in his sleep with the switches re-activated.
- Why the Robinsons need a highly complex Robot to act as a jumper cable in the Cosmic Energy Grid instead of a simple piece of conducting wire is never fully explained.
- Zetan Galaxy Law Enforcement sends two androids to the Robinson camp. They have the same silver faced make-up and silver costumes used other Irwin Allen television productions. It was never made clear if these androids were the same alien species as, for example, "Kidnapped in Space.".
- The Jupiter touchdown on the planet is anything but smooth -- it bounces twice before settling.
- No environmental tests were conducted before Will was allowed to venture out of the spaceship.
- Where did the Robot get the shovel?
- Where did the movable stone door on the cave come from?
- If the female Robot was aware she was standing on a disintigrator, why didn’t she move?
- If the Robot had the knowledge of how to make his girlfriend “nice” by omitting her “evil components,” why didn’t he do so when putting her together the first time around?
- What happened to the female robot after Robot remade her?
- The female robot is the same robot used in "The Ghost Planet," except painted blue instead of red.