As they are rehearsing a play, Dr. Smith, Will, Penny and the Robot encounter a creature that appears to be suffocating. Dr. Smith refuses to help out of fear, but Penny helps the creature. The alien puts himself in a humanoid form so that he may breathe the atmosphere. He says he is Mr. Arcon.
Penny takes Arcon back to the Jupiter 2 to rest. Later Arcon gives Penny a gift, which he says she must guard with her life. He tells her that as long as she wears the belt, she will be safe. He disappears, and it is not long before a group of aliens turn up, looking for the belt. They inflict immense cold on the planet as a way of trying to get the belt, but Penny refuses despite pleas from Dr. Smith. Meanwhile, Dir. Smith makes a copy of the belt and tries to give the fake to aliens. They are angry, but still want his help. They promise to return him to Earth should he get the belt.
Dr. Smith and Penny go through the alien's machine, where they have created a world from Dr. Smith's memory, which will trick them in to thinking it is Earth. Penny gives up the belt, and in the meantime, Arcon returns to the spaceship looking for the belt. Realizing she is in trouble, Arcon goes to the place where Penny is and defeats the aliens. He is angry she gave it up, and believes she did it selfishly. She protests and claims she did it because they threatened to hurt her family. Arcon takes this thought and leaves.