Unfortunately, it seems that most of the answers here have derailed into something I specifically have not accepted since my first comment in the OP. Namely, people have been suggesting that I should switch from the mind-controling device into more normal ideological propaganda. Unfortunately, this falls outside the scope of the question, for these reasons:
- Propaganda doesn't work on everyone. Its mechanism is psychological and societal. I wanted a purely biological mechanism with high penetrance, irrespective of education, ideology or opinion
- The purpose of this question was to test a particular TV trope. Propaganda falls in another genre altogether, the dystopian genre
- In that particular TV trope, the machine is switched on and off at will (in fact, it's the super-hero's job to turn off the device, imediately saving everyone)
- A mind-controlling device is cooler (yep, that's an objection I have, too)
Only Amadeus and a4android's answers tried to address the question, so I'll try to answer my own question piecing together what I liked most on both answers and my own knowledge on the matter.
First, I have to agree that this device is not possible using present-day technology. But I have said that near-future tech was fair game. I don't think that nanomachines that can be introduced in the organism for medical purposes are so distant in the future so as not to allow the use of nanobots on this question. The fact that we still know so little about brain function is a more significant hurdle... but maybe we can circumvent it with as little handwave as possible (which was the purpose of the question all along).
Second, I'll take into account some objections in Amadeus' answer, namely:
Objection a. 25% of the population is non-hypnotizable; The device should work on those people too
Objection b. People should be susceptible only to orders issuing from the leader, not from other people
Objection c. The instructions should be given in a way that everyone will be able to understand, irrespective of other factors, like language or IQ
Having said this, I've figured out four possible approaches, all using nanomachines. These could be spread by the supervillain in a city during months or years and could remain dormant until the switch was activated.
Note, these four approaches are displayed in increasing order of personal preference. Meaning I like approach #1 less and approach #4 more
#1. The Puppeteer
In this approach, we have different types of nanomachines that invade different parts of the human brain. Some nanomachines would be stimulatory and others would be inhibitory. The stimulation / inhibition could be done through electrical impulses or microinjections of selected neurotransmitters.
The inhibitory nanomachines would go into the frontal lobes, where the higher funtions of the brain (like will, abstract reasoning and personality) are located. By selectively inhibiting the frontal lobes, the nanomachines could effectively shut down all voluntary control from the victim.
The stimulatory nanomachines would then take hold. These would nestle on various parts of the primary motor cortex (PMC). We have a pretty reliable map of the PMC, such that we know which body part will move if an electrical stimulus is given to a specific region of the PMC.
The nanobots could then be programmed with simple movements, like walking a specific number of steps or getting up if the host fell down.
This means that the supervillain could effectively turn people into mindless puppets.
All people would be affected, regardless of language or hypnosis susceptibility. Also, since everyone but the supervillain would be unconscious, there would be no one else to order people around. Besides, only the supervillain would have control over the nanobots.
- This is an over-complicated approach. The supervillain would have to fabricate a myriad of different nanomachines and those would have to be allocated to several different parts of the brain. All those nanomachines would have to be perfectly synchronised on the same individual, so as to obtain the desired effect
- Even if everything went smoothly, the resulting movements would be uncoordinated and jerky. Coordination relies on the integration of the PMC with secondary motor centers. We still know little about how those work, so I think that nanomachines located in the secondary motor centers would lead to too much handwave. The puppets would not have fine motricity, so the tasks they could do would be very limited. For example, they could not do something as simple as picking an object from a table... it would be like trying to grip a prized toy with those claws from the amusement parks
- Also, there would be no way for the nanomachines to integrate their motor stimuli with the sensory information coming from the host, like the eyes or the proprioception receptors. The nanomachines could move people around to a specific location using GPS, but wouldn't be able to detect unpredictable obstacles. The human puppets could fall on ditches or trip in one another.
#2. The Poisoner
Maybe the supervillain could use something like approach #1., but only in part. Namely, the part about inhibitory nanomachines in the frontal lobes that would shut down the conscious person.
The supervillain could then enter town and give each person a dose of the mind altering drugs that form the basis of Amadeus' answer. If, by biological variance, some slaves did not respond to the drugs, the supervillain would only need to shut them down again using the nanobots... and then kill them.
- Time consuming. The supervillain would have to find ways to drug everyone in town.
- Some people could escape, if they would be hidden from plain sight or stuck in inaccessible places when the device was turned on
- Even more complicated than approach #1.
#3. The Messiah
Since approaches #1 and #2 are so complicated, maybe we could focus the nanobots on a single spot. The limbic system is responsible for lower brain functions, namely related to emotions or primal behaviors. Also, it is well accessible... the nanomachines could crawl up the nasal cavity and then lodge themselves on the lower parts of the brain through the pores of the ethmoid bone.
By stimulating this regions of the brain, the nanobots would cause all kinds of heightened emotions, like fear, rage or lust. These emotions would be so strong, that they would override any rational thought, like Amadeus says in his answer. But no matter how much the victims would indulge in violence or sex, their urges would not be satisfied as long as the nanomachines were activated.
It would require a level of self-discipline akin to a shaolin monk to be able to resist this kind of overwhelming sensations.
Enter the supervillain. He would place a tracker on him and the nanomachines would be programmed to change their pattern of stimuli accordingly to the GPS calculated distance from the villain. As the supervillain approached his victims, the nanobots would cease their fear / rage / lust stimuli... and could even induce pleasurable stimuli.
People would eventually be conditioned to view the supervillain as a kind of liberator. Once this was accomplished, he could tune his stimulation with finer detail, inducing pleasurable stimuli as a reward for good obedience and unpleasurable stimuli as punishment for disobedience.
Eventually, people would be conditioned to obey the leader blindly, almost in a pavlovian way. The nanomachines would then be dispensable. Propaganda and regular brainwashing would suffice, for trust on dear leader would be a non-issue.
The pre-conditioned victims would have unpredictable and erratic behavior. Many slaves would die before the supervillain could do anything about it. In fact, there's no guarantee that the supervillain would not be killed previously to the conditioning (maybe in a car crash or something like that).
#4. The Wish Seeder
This is my favorite approach. Unfortunately, it is also the one that requires the greatest amount of handwave.
I inspired myself in a4android's answer about synaptic mapping. However, synaptic mapping is pretty much beyond our reach in the near future. Also, the synaptic mapping of each individual is different, so this concept would not be able to sustain widespread mind control.
However, as approaches #1 and #3 show, we don't need a detailed synaptic map. We can focus on specific regions of the brain... what if we focused on the frontal lobes?
As I have said, the frontal lobes of the brain are the ones associated with the higher functions: will, abstract thought and personality.
Unfortunately, artificial hardware and neurological "hardware" are still largely incompatible, because they have different languages. We still do not know how to translate electronic language into synaptic language.
However, we already have rudiments of this translation in the form of neuroprosthetics and brain-computer interfaces.
Let's imagine that the supervillain has perfected an interface that can translate an instruction from a computer into a thought via a nanotransmitter lodged in the frontal lobe of the victim.
The leader could then type the order in the computer and the person would find that instruction perfectly reasonable and/or desirable.
This is the best, because unlike all the previous approaches, the slave can perform complex tasks and behave in a predictable and reliable way.
Needs a high amount of handwave, because we can't explain the details of how such an interface would function. Also, in order to account for objection c, the computer instructions shouldn't be given in the form of words in a specific language, but in the form of images or such. Still, an interesting concept that could be perfected with some more imput from other commenters.