Professor Robinson and the Robot land the Space Pod on a planet that the Jupiter 2 is passing. John has to kill a dangerous monster moments after the landing. By doing this he has interfered with a hunting contest that is being used to test an alien named Megazor to determine his worthiness to be the ruler of his planet, Zon. Megazor captures the Professor, and decides that he has no value as prey in the hunting contest. But, after John kills a dangerous alien in a duel, Megazor changes his mind and tries to use the Professor as prey in the contest.