While rehearsing a play, Penny, the Robot and Dr. Smith are interrupted by a hideous alien. Dr. Smith flees, but the Robot announces that the alien is not hostile. Penny befriends him, and the alien transforms itself into a life form that can exist on that planet. He introduces himself as Arcon, and when he reveals he is being pursued by the Saticons seeking a powerful amulet he holds, Penny offers him sanctuary at the Jupiter 2.
John and Don are away, but Maureen is happy to give Arcon refuge. Soon, though, the Saticons show up to demand the the release of Arcon. When the Robinsons refuse, the Saticons reduce the temperature in the ship to below freezing. Arcon, impressed by what the Robinsons have done, concludes that they are virtuous and gives the amulet he holds to Penny. He warns her never to surrender it, for the Saticons would use it for great evil. He then flees and the ship is returned to normal.
Dr. Smith goes to speak to the Saticons intending to make a bargain with them. The Saticons agree that if Smith acquires the amulet for them, they will send him back to Earth through their molecular transporter. Smith has the Robot make a duplicate of the amulet but the Saticons are not deceived, and threaten him with the consequences of failing in his bargain with them. The Saticons actually have no intention of keeping their end of the bargain either; they have prepared a simulacrum of San Francisco’s Chinatown on a nearby asteroid to fool him.
Dr. Smith tries to convince Penny to give him the amulet, but she refuses because she must keep her promise to Arcon. Penny is confused, but Arcon appears to reassure her and to tell her to do what she feels is right.
Smith is finally able to convince Penny to surrender the amulet. As a sign of good faith, the Saticons send Penny, Smith and Robot through the molecular transporter to the simulacrum. The Robot immediately warns that it is an alien planet, but Smith will not listen. The Saticons arrive to demand the amulet, which Smith gives them, but Arcon arrives to rescue them and take them back through the molecular transporter before they are destroyed.
The entire situation was set up as a test of Penny’s character, and Arcon is disappointed at her failure. When he realizes Dr. Smith was to blame, he wants to send him to a dead star, but Penny and the Robinsons convince him to relent.
- The Saticons have returned to harass the Robinsons. (They were actually destroyed by the Robot in "Wreck of the Robot"; hence they did not return; they are new Saticons.)
- Mr. Arcon's original form is the same as Mr. Keema from The Golden Man, and before he becomes human, he assumes the form of the frog alien from that same episode.
- Asteroids are far too small to hold an atmosphere or to have a significant gravitational field. The fact that Penny and Dr. Smith can breathe and move normally must therefore be due to the designs of the Saticons.
- Upon his arrival, why did Arcon knock at the prop door?
- When Penny first speaks to Arcon, the Robot says he will remain nearby in case she needs him. We then however, see him back at the spaceship with Will.
- How did Debbie know to “select identifiable objects and then return?”