When Dr. Smith accidentally blows out a power relay, the Jupiter 2 is left with insufficient energy to meet its needs. One consequence is that the Robot cannot be recharged; Will is particularly upset because, when the Robot’s power reserves are completely used up, it will cease to function—effectively, the Robot will die.
In an attempt to save Will and the other Robinsons from further pain, the Robot records a message to them before leaving camp to go off and die. He travels as far as he can before his energy is so low that he collapses in the Valley of Shadows.
When Will discovers that the Robot has left, he sets out with Dr. Smith to find him. They track him down, only to find he has grown to enormous proportions because of radioactive gas found in the area. Undaunted, Will sets about repairing the Robot by actually climbing inside and making his repairs that way.
With the Robot’s voice to guide them, they avoid his self-defence mechanisms and arrive at the source of the Robot’s malfunction. By that point, the Robot’s power is almost exhausted, but Will does everything in his power to save him, despite Dr. Smith’s repeated demands to return outside. At one point Will even manages to revive the Robot when his diode timer, his heart, ceases to function.
The next morning, John and Don are searching for the missing members of their party. When they find the gigantic Robot with an access panel on his base open, they deduce Will’s plan and decide to go inside to find the boy and Dr. Smith. Meanwhile, Will discovers how to return the Robot to his original size and he begins to shrink quickly.
Smith, Will, John and Don are almost trapped as the Robot grows smaller and small. All escape in time, though, except for Will. John is frantic with worry and he and Don attempt to keep the Robot’s access hatch open with a log to allow Will to make it out. At the last moment, with barely enough room to spare, Will’s head appears and the drag him out. After their close call, and happy to have the Robot back, the Robinsons recharge him fully.
- Many props from other episodes are used to represent the Robot's internal workings. Most noticeable is the Purification Arch prop from The Colonists. In this respect the episode could be what is called in the TV industry a "bottle show" - a script that can be filmed cheaply as it uses existing props, sets, special effects, and a minimum cast. Bottle shows are typically kept in reserve in case a committed script becomes undoable and something is needed at the last minute or in order to bring production costs back under control.
- Knowing how completely inept Smith is, why did John trust him with an important job?
- We see Smith eating an apple and a banana. However, in the episode “Wish Upon a Star,” it is clearly stated that apples are rare items that the Robinsons do not have. And if they can’t get apples, it’s very doubtful they could get bananas either....
- When Will finds the Robot, he tells Robot that John “changed his mind” and is willing to give Robot the power he needs to survive. But the next morning at the Jupiter 2, John himself states that he has remained steadfast and has NOT changed his mind.
- The Robot has been powered down many times before, and in several cases, completely disassembled. Why did he not die then?
- The Robot has been turned completely off, and later, turned back on again many times. Why didn't the Robinson's just turn the Robot off, then turn him back on again after they were back up to full power?