Superboy; Jor-El (flashback); Lara (flashback); Ha-Kor (intro, in flashback); Gra-Mo (intro, in flashback); Gro-Mo's two unnamed assisants (intro, in flashback); Phantom Zone inhabitants [Jax-Ur; Dr. Xadu; Professor Vakox; six unnamed prisoners] (all in flashback)
Jor-El invents the Phantom Zone Projector as a more humane way to punish criminals than sending them into space and successfully submits it into a Science Council competition. The disgruntled loser of the competition, Gra-Mo, takes over Krypton's robots to take his revenge, and ironically becomes the last criminal to be punished in the old way. Months later, after many convicts have been sent into the Phantom Zone, the prisoners try to take over Jor-El's mind with mental telepathy, so he blasts the Zone Projector into space with other forbidden weapons.
Superboy; Martha Kent; Jonathan Kent; Lana Lang; Chief Parker; Superboy robots; Gra-Mo; Gro-Mo's two unnamed assisants; Phantom Zone inhabitants [Jax-Ur; three unnamed prisoners]
Gra-Mo and his accomplices land on Earth and, with the help of Jax-Ur in the Phantom Zone, plot to destroy the son of Jor-El. Using a robot, they try to convince Superboy that he has broken his vow against taking a life, but Superboy sees through their ruse and banishes them to the Phantom Zone.