After a wild wind storm destroys the Robinsons crops, the family must go on short rations supplemented by protein pills. No one is very happy about this, especially Dr. Smith, who swears he is going to drop dead of starvation. As it happens, the storm was no accident. It was caused by a crafty merchant who has landed on the planet and intends to do business with the Robinson family. By ruining their supply of food, he hopes that they will be more inclined to trade with him. The Robinsons, of course, are unaware of this.
Will and Dr. Smith discover the trader's shop while on a walk. The merchant approaches them and introduces himself as the Trader. He offers to show them his wares, and he has many interesting items which impress Smith and Will both. Smith informs the Trader that they need food, and the Trader shows them that he has plenty. He will not, however, give it away for free, and will only accept useful items in exchange.
Smith goes back to the Robinsons and tells them the news, but John and Maureen say they cannot spare anything for trade and that they will be able to manage on their own without the Trader’s help. Even when the Trader arrives at dinner time to tempt the Robinsons with a turkey, John still refuses his offer. The cruel Trader goes back to his shop and causes another wind storm to make the situation even worse.
The next day, Dr. Smith and Penny go to see the Trader. Smith has convinced Penny to exchange her tape recorder for some food, but the Trader refuses the offer, saying he does not conduct business with children. He lets Penny keep her tape recorder and gives her a stick of candy. Penny is quite happy and goes back home, leaving Smith alone with the Trader. The Trader convinces Smith to exchange the Robot for food.
That night Dr. Smith takes the Robot on a walk with him and they head toward the Trader’s shop. The following morning, when Will goes looking for the missing Robot, he finds him chained up in the Trader’s store with a price tag stuck to his chest. The Trader informs Will that the Robot is now his property since Smith sold him. Will is shocked and runs home to tell the others. The whole family is horrified and outraged at Dr. Smith, and they vow never to speak with him again. Eventually Smith can bear the silent treatment no longer and goes to the Trader to get the Robot back. Smith has no money or items to give in exchange, and the Trader refuses to give back the Robot on credit, but he does come up with another solution. Smith can have the Robot if he offers himself in exchange. The Trader assures him it is only an informal trade for book-keeping purposes and promises that he will not collect for two hundred years, and since Smith knows he will not live that long, he agrees and signs the contract.
Doctor Smith takes the Robot back to camp with him, and everyone is very glad to see him. The very next day, the Trader shows up at the Jupiter 2 and says he has decided to collect early and that Smith must now go away with him to be sold at a galactic marketplace. Smith refuses, but the Trader uses the electronic contract to put the doctor into a trance. He lures Smith away to his spaceship, and, determined to help, Will follows. Will helps Smith out of the trance and they escape back to the Jupiter 2 with the Trader’s dogs in pursuit. When the Trader attempts to take Smith again, the Robot intervenes. He says that the Trader is unethical for tricking Smith, and reveals that the devastating storms were his doing. The Robot blasts the electronic contract and Smith is set free. The Trader and his dogs return to their spaceship and leave.
- The language translation device that the Trader uses is the same prop that was used as the "Thought Machine" in "Wish Upon a Star".
- When the Trader's dogs come running after Will and Smith, during the scene where they approach the Trader's camp for the first time, one of the dog handlers looks up from behind a prop rock, then lowers his head very quickly.
- The spaceship used by the Trader is the same spaceship used by Alonzo P. Tucker in "The Sky Pirate"
- When John and Maureen are examining a tray of food found hidden on the ship, the fruit makes a clinking sound when rearranged like it is ceramic.
- Is Doctor Smith really willing to risk his life for that dumb painting? I would have thought his own selfish cowardice would have sent him scurrying for shelter right away.
- It is assumed the tank that almost crushed John and Smith was supposed to be full of water, but it rattled so loudly it was obviously completely empty.
- The Robot gets very angry at Smith, declaring “You insulted me!” Doesn’t Smith insult the Robot multiple times on a daily basis? What was so different about this time that made the Robot upset?
- Doctor Smith actually traded the Robot for food? Not only is that awful beyond even Smith’s capabilities, but it’s also very stupid. Certainly he would have known everyone would find out the very next day, and that they would be furious. And to make things even worse, he didn’t even share the food with them! Apparently Smith is absolutely incapable of thinking ahead.
- The Trader eventually removed the Robot’s power pack, but when Will first saw the Robot in the shop, Robot was awake and aware. Why exactly was Robot just standing there? Why didn’t he blast through his chains and escape? He gives the Trader and his dogs a good zap later on, so why didn’t he just do so in the first place and refuse to be merchandise?
- When trying to convince the Robot to come for a walk, Smith says that the Robot needs the exercise because he is getting “Roly-poly from standing around so much!” I really hope Smith was just teasing, because my brain can’t accept the idea that the Robot would be capable of getting fat.
- Why does Doctor Smith run for about thirty seconds and then collapse on a rock and give up? It’s not tag, Smith... it’s your life! Run!!
- Will foils the Trader’s guard dogs by tossing them a stick. Earlier the Trader told us these were specially trained dogs that he paid a great deal of money to acquire. Why would sophisticated space dogs get distracted by a stick?
- It is amazing that the Robot was able to actually catch and throw a ball! That must have been a lot harder then it looks, grabbing a round object with a pair of hooks. Very cool, Bob May!
- In the first scene when John, Don, Penny and the Robot are entering the spaceship to escape the storm, we see the robot about to topple backwards as he goes up the ramp.