The Jupiter 2 prepares to make a landing on an alien planet so they can make much needed repairs. Before they do so, John and the Robot go out in the space pod to make an initial assessment to be sure it will be safe to land. A force-field hits the pod, making it go out of control, but it manages to land on the alien planet with minimal damage to pod and crew.
When John and the Robot get out of the pod to look around, an alien beast attacks them. John blasts the creature with his laser. Seconds later, a blue-skinned alien named Megazor appears saying that John has trespassed on his private hunting grounds. Megazor takes John and the Robot prisoner as punishment for interfering in his sport.
When the Jupiter 2 is unable to communicate with John, the Robinsons decide to land on the alien planet to search for him. Smith doesn’t want to and tries to take control of the ship. All he manages to do is to send it flying out of control and damage several important systems. The Jupiter 2 crash lands, but luckily no one is seriously hurt. Dr. Smith and Will scan the area for John and the Robot, but are menaced by a rock monster until one of Megazor’s guards captures them.
Meanwhile, Megazor makes John fight an invisible, poisonous monster to determine whether or not John would make suitable prey. John defeats the monster and Megazor is impressed, deciding that the professor would make for very interesting game. Will and Smith are then locked into the cage with John. As Megazor prepares for the hunt, John, Will, Smith and the Robot make an escape. Instead of running away with the others, Smith foolishly hangs back and is captured all over again.
The others back it safely back to the Jupiter 2, but realize Smith is not with them. Megazor sends radios them, saying that if John does not return to participate in the hunt, the alien will hunt Dr. Smith instead. Knowing that Smith wouldn’t stand a chance, John bravely returns and Will secretly follows after him.
John is dressed for the hunt in a spiffy silver suit and given a five minute head start before Megazor begins to hunt him. Megazor holds Will hostage to ensure the professor’s co-operation, and Will is allowed to observe the hunt. John runs for a long time, but inevitably Megazor draws in for the final blow. John falls into quicksand and Megazor is about to kill him when Will intervenes and begs the alien to spare his father’s life. Will distracts Megazor long enough for John to escape from the quicksand. John and Megazor fight and the Megazor is killed. The computer then says that there was no need for them to destroy Megazor, and that the planet will look for a new heir. He offers John a chance to try for 500 points, having gotten 150 from killing Megazor, but John refuses saying he has too many Earthling defects. The computer then leaves, and John and will walk back to the ship. Along the way, the robot frees the still caged Dr.Smith.
- Most of the location shooting was done at Fox's Malibu Ranch.
- The arms and body of Megazor's computer were repainted and used as Tybo's water pump in "The Great Vegetable Rebellion".
- In typical LiS fashion, the Robot claims the force field around the planet is energized by "solar deflection." This term is actually used with respect to the bending of light caused by the gravity of a star or other massive object distorting spacetime. The experimental confirmation of this effect in the early 20th Century helped support Einstein's Theory of General Relativity.
- There is usually a very extensive environmental check of air, water and soil before the Robinsons venture out onto a new planet. This time, other then asking the Robot a couple of brief questions, no real checks were done at all.
- What is the “radar-scope” Will and Smith were using? What does it do and why have we never seen it until now?
- John orders the Robot to burn out one of his systems, and Robot strongly protests the idea, saying that ‘self-inflicted damage is contrary to my programing.’ However, in the episode “The Challenge” the Robot damaged himself purposely just to avoid having to talk to Smith. And in the episode “War of the Robots” the Robot actually tried to burn out his primary memory banks and kill himself.