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As the episode opens, the Enterprise is approaching a planetary system from which they have received a distress call. Spock reports that sensors show no life in the system - apparently some 4 billion beings have been killed. They rapidly dismiss conventional explanations, especially since contact had been made with the system just a week before. Kirk's reaction to 4 billion deaths is understated, but believable - he leans on the console for support as it sinks in. Unfortunately, no one really seems to keep this huge loss in mind later on in the episode.
The ship soon comes under attack by some kind of weapon that travels at warp 15 (so it cannot be outrun) and packs the equivalent effect of dozens of photon torpedoes. The deflector shields protect them through several hits; return fire is completely ineffective. As a last recourse, Kirk tries to make contact, transmitting his name and their peaceful intentions. Of course, their peaceful nature might have been more believable if they'd tried sending the message before they returned fire, but I can't really fault their return attack under the circumstances.
The attack ceases, and the aggressor makes contact, rapidly assimilating the Enterprise's language records. The object, calling itself Nomad, agrees to be beamed aboard. Spock has already ascertained that the object is a cylinder approximately 1 meter long.
A metal cylinder with some attachments is exactly what is beamed aboard. After some abortive conversation, Kirk and company realize that the object is a robot named Nomad, not some type of spacecraft with tiny beings inside. This whole exchange is kind of amusing, since we never really see aliens as tiny as they imagined these ones were.
While Kirk provides Nomad with some information to keep it occupied, but which is not too revealing, Spock checks the historical records for an Earth probe named Nomad. Nomad begins calling Kirk "the Creator" and refers to all the other humans (and Spock) as Kirk's biological units. Spock asks Kirk to leave the room so they can discuss his findings.
Spock tells Kirk and McCoy that Nomad does in fact seem to be the Earth probe Nomad, with some modifications. The probe had been thought to be lost after a collision with a meteoroid; apparently it later encountered some other kind of being or spaceship and was repaired. Nomad's original purpose was to search for new life. The current Nomad is far more powerful than the original, and obviously is following a program of aggression, given its destruction of an entire planetary system. Nomad has mistaken Kirk for its creator Roykirk; Spock encourages Kirk to keep up this deception.
Meanwhile, Nomad has gone to the bridge. Uhura is singing, so it approaches her and asks her about music. In order to learn about it, he does some kind of scan of her. Scotty thinks she's under attack and rushes Nomad; Nomad zaps him. (This is very consistent for Scotty - always rushing in to help a woman, such as in "Who Mourns for Adonais?") Kirk, Spock, and McCoy arrive; McCoy pronounces Scotty dead. Uhura is conscious but unresponsive. Both are taken to Sickbay.
Nomad is puzzled by Kirk's anger at its actions; he offers to repair Scotty. After assimilating computer files on human anatomy and the brain, it goes to Sickbay and brings Scotty back to life. It says it cannot "repair" Uhura, however - Uhura's memory has been erased, but there is no physical damage. McCoy and Chapel begin a program of re-educating Uhura.
Spock decides that a close analysis of Nomad is necessary to figure out how to neutralize it; Kirk orders Nomad's cooperation. However, conventional sensor scans do not provide Spock with enough information, so he mind-melds with it.
This is a very interesting idea - can Spock mind-meld with something that is not alive? Is the ability to participate in a mind-meld a de-facto test for life? I cannot believe that Spock could mind-meld with something that didn't have a consciousness, so this shows that Nomad is in fact alive. In fact, Nomad actively participates in the mind-meld, keeping the "connection" going even after Spock tries to stop. Kirk must order Nomad to stop.
In private, Spock relates Nomad's story: some time after the meteoroid collision, Nomad met another, very powerful space probe. For some reason, the two probes combined structures and programming. The other probe's instructions were to acquire and sterilize soil samples for study. The resulting probe, calling itself Nomad, believes its purpose is to find perfect life, and if that life is not perfect, then it must be sterilized. Spock believes that the only reason the people on the Enterprise are still alive is because Nomad thinks Kirk is its creator.
Nomad, meanwhile, keeps trawling through the ship and studying it. When security guards try to corral it, it vaporizes them. Kirk confronts Nomad, and in frustration reveals that he is also a "biological unit". Nomad states that it must re-evaluate the situation, and proceeds to ransack the computer records to read Kirk's file.
Spock is alarmed by Kirk's revelation to Nomad, and surprisingly Kirk admits that it was a "foolish mistake". It's a sign of their strong relationship that Kirk can stop being the perfect captain, at least for a short time. They both realize that they have only a short time until Nomad decides to kill them all. Even worse, Nomad referred to returning to Earth - presumably once it's there, it will sterilize the planet.
Kirk confronts Nomad again in engineering. He questions Nomad purposely about Nomad's directive: that anything imperfect must be sterilized. Then Kirk reveals that he is not actually Nomad's creator, so Nomad has made an error. He repeats Nomad's directive, emphasizing that since Nomad is imperfect, it must be sterilized. Fortunately, Nomad apparently does not know that humans can lie, and takes Kirk's revelation as fact. It quickly begins to malfunction. This one of the best cases of Kirk "logicking" a computer to death, because assuming Kirk cannot lie, the logic is irrefutable. It's not based on more subjective judgments, such as "the good of the body" like in "Return of the Archons".
Kirk, Spock, and Scotty quickly take Nomad to the transporter room and beam it out into space far from the Enterprise. Within seconds, it self-destructs.
We end, as always, with a light moment. In this case, it's incredibly forced: Kirk jokes that he was Nomad's mother. However, they've just killed a unique being (albeit one intent on destroying humanity), and 4 billion people have been massacred. Perhaps we don't need a joke here.
This episode centers on an idea that will be reprised in Star Trek: The Motion Picture: an Earth probe that gains sentience and is returning to Earth. It's not an uninteresting idea, but in this episode it is not executed that well. The pacing is quite slow; even Nomad's motion through the ship is slow. And Kirk and his crew make some dubious decisions.
Kirk is remarkably slow in catching on to the idea of pretending to be Nomad's creator and putting it to good use - I kept wishing Spock would prompt him more. One thing that bugged me especially was an order Kirk didn't give. He knows that Nomad has no regard for human life, and soon learns that Nomad will follow his orders. Why didn't he order Nomad not to harm any of the people on the Enterprise? That might have saved Uhura, Scotty, and at least one set of security guards.
I was also bothered by Kirk and Spock's failure to brief other key people, such as McCoy, Scotty, and the relevant security people, on Nomad's capabilities and how they were deceiving Nomad regarding the "Creator". Of course, this failure to disseminate knowledge is endemic to the series, and I suppose it would be boring to see a bunch of scenes where people are briefed.
No mention or speculation is made on how Nomad (being only a meter long) was able to generate as much energy as it did during the attack on the Enterprise, let alone its ability to wipe out an entire planetary system. Frankly, this was probably for the best, since any explanation would probably sound really silly today. However, I am still left wondering why the "other" that Nomad merged with was so incredibly powerful when all it was supposed to be doing was gathering soil samples.
Uhura's loss of memory is a huge event for this character, and really should have ramifications farther on in the series, but of course that's not how TV series were in the 1960s. I supposed that in principle she should be able to be re-educated, but I can't buy that she could make progress so quickly. And how did she somehow know her native Swahili when she was reading first grade English?
We get another slam at women, when Nomad declares Uhura to be "a mass of conflicting impulses". Presumably Nomad would have said the same thing about a human male, but of course the situation wasn't set up that way. I'm sure this was supposed to be funny, despite the fact that the woman in question was just assaulted.