The Men in Grey, also known as the Grey Gentlemen, are the antagonists of the German novel Momo by Michael Ende. They also appear in the cartoon series and the live action movie based on the book. They are a race of paranormal parasites described as time thieves.
Though their number is huge and they are a deadly threat to all that makes human existence worthwhile, no humans ever become aware of them, except for the little girl Momo, some other children and a few of their adult friends.
As for the origins of the Men in Grey, it is simply stated that they came into being because humans allowed them to do so. It is said that people would also be able to destroy them (presumably by denying them time), but since humanity is almost completely ignorant of them, they can carry on their sinister vampiric existence.
Their near-human appearance is said to be a disguise only (not necessarily implying that they have a different "true" form, but simply that this is the shape most useful to them). They look like grey-skinned, bald men, dressed in grey clothes, and wearing grey bowler hats. For a very good reason, they are also incessantly smoking little grey cigars, ever lighting a new one with the butt of the former.
The size of their organization is not stated, but there are hints that they at one point numbered in the thousands, and pretty much dominated an unnamed human city (apparently in Italy, since Momo's friends all have Italian names). Even then, most humans never became aware of the paranormal invaders.
In the novel, they Men in Grey keep trying to steal time by manipulating people with devious promises or deals (for example: "Time's money, save time and you get rich"). The Men claim to represent a strange institution called the Timesavings Bank, where people can supposedly deposit time and withdraw it later, with interest. However, all who make a deal with the "Bank" subsequently forget about the Men in Grey, and are only left with a strange obsession that they must "save time" by becoming more efficient.
However, when a Man in Grey visits Momo, it is as if he is bewitched by her unique talent for listening, and her special gift pulls the truth out of him: "All the time [people] save, they lose to us. We drain it off, we hoard it, we thirst for it. Human beings have no conception of the value of their time, but we do. We suck them dry, and we need more and more time every day, because there are more and more of us. More and more and more. . ."
Now that Momo knows the truth, she is a great threat to the Men. They try to make several deals with her to keep her from stopping them, but they all fail.
However, the Men in Grey do get to the few adult friends she had told about the time-thieves. One is rendered harmless simply by granting him the fame and wealth he had dreamt of. Finding his new existence hectic and meaningless, he finally tries to gather courage to reveal the Men in Grey to the world. He instantly gets a phonecall from them, curtly informing him that this would cost him everything. He gives up the plan and loses whatever remained of his self-respect.
Momo eventually gets in contact with the kind if enigmatic "Professor Hora" (implied to be a quite superhuman entity, and possibly the master of life and death; he answers evasively when Momo asks if he is, in one sense, Death). The Master-Supplier of Time, Hora is the ultimate enemy of the Men in Grey, who want to get to him and steal all of time directly from the source. Hora reveals the true nature of time to Momo: The time he sends to the human race manifests as hour-lilies growing in each person's heart, each flower unique and ineffably beautiful, but only lasting for one hour. But if humans inadvertently allow the Men of Grey to enter the heart (by making deals with the "Timesavings Bank"), the Men will steal ever more hour-lilies and turn them into the little cigars they must constantly smoke to remain in existence.
Unfortunately, the Men in Grey are able to sneak behind Momo when she goes to Professor Hora for the second time, so that she inadvertently reveals the way to him. The Men in Grey proceed to lay siege to Hora's dwelling. He then decides to go to sleep, which will cause time to stop altogether, and drive the Men in Grey to extinction when they cannot steal more hour-lilies. However, to reawaken Professor Hora and prevent the world from standing still forever, someone has to release all the stolen time hoarded by the Men in Grey. Momo accepts the task and receives one hour-lily from Professor Hora to keep her going for 60 minutes after time will freeze for all others.
Professor Hora goes away, and all of a sudden the entire world appears frozen in time. The Men in Grey panic and flee back to their hidden underground lair. It is implied that the vast majority of them perish on their way: When time froze, their cars would not move anymore, and they could not reach their lair on foot before their cigars were consumed. Momo follows the survivors and secretly watches as they further decimate their own number to stretch their supply of time as long as possible: Only six Men in Grey are allowed to go on existing. (They halved their number four times, implying that there were originally 96 survivors that had made it back; one of them described this as a mere "handfull" compared to their original number.)
Momo sneaks up to their vault, and by touching the door with the hour-lily, she is able to shut it. Cut off from their store of stolen time, the last six Men in Grey now face the extinction of their entire race as soon as their cigars are finished. Their furiously pursue Momo to get to her hour-lily and open the door again. Four perish one by one, and when the last two finally manage to corner Momo, they start fighing among themselves over who will have her hour-lily. One disappears when the other knocks the cigar out of his mouth, and as the very last Man in Grey demands the flower from Momo, he also loses the tiny butt remaining of his own cigar. In his last few seconds of existence, he begs her to give him the hour-lily, and as she refuses, he remarks that it is good it is all over. Thus the Men in Grey perish with a modicum of dignity.
Using the very last minute she has before her hour-lily will crumble, Momo opens the vault again, releasing the millions of liles inside. They return to their human owners and go back to their hearts, causing time to start again (with nobody perceiving that it ever halted). Professor Hora awakes from his sleep, Momo is reunited with her friends, and all is well.
The Men in Grey appear more or less identical, and their numbers quickly increase. It is however never made clear how new Men are created. Since there are no Women in Grey, they are not a gendered race at all, but merely imitate the form of human males.
During a trial, one of them describes himself as eleven years old even though he appears like an adult. He also states that he has been working as an agent from the moment he came into being (a piece of information dismissed as irrelevant by his judges, since this is a matter of course among the Men). It would therefore seem that the Men have no equivalent of a childhood. They come into existence as "full-sized" individuals, presumably able to speak and with basic knowledge of the world, and instantly ready to start serving the organization they belong to.
Much like ghosts, the Men radiate cold, and temperature quickly drops when they enter a room.
The Men in Grey are likely potentially immortal. Nothing is said about any of them appearing older than others, as if subject to aging and eventual death. It is never established whether they would be vulnerable to bullets or physical force, though they seem to be actual physical entities (as seen from the fact that they can drive cars). Near the end of the novel, some of them display cuts and bruises, implying that their bodies can sustain damage and that any regeneration is not instant.
The only known way to kill them is to keep them from smoking their cigars, because this is how they consume the stolen time that keeps them going. The cigars are made of the petals of dried hour-lilies: Hour-lilies are representations of the time of a human, and grow in the human heart. When a person enters into the dubious "time-saving" deals offered by the Men in Grey, he or she somehow makes the heart accessible to the Men, who will then steal ever more hour-lilies. From the perspective of the humans so affected, their days suddenly seem short and hectic, and they no longer have time to really enjoy anything in life.
Deprived of his cigar, a Man in Grey will literally fade into nothing within seconds, leaving no trace. This implies that their clothes, shoes and hats are also made of stolen time, and are really part of their "bodies". Hats, and so possibly other articles of clothing, do not have to be in physical contact with the (main) body to go on existing: The Men are sometimes described as removing their hats. As shown in the life-action movie approved by the original author, even hats not on the head of their owners will disappear if the Men they belong to perish. This implies that each article of clothing maintains a non-localized connection to the owner and is dependent on his continued existence.
Since the Men in Grey must incessantly smoke to maintain their existence, it can be inferred that they have no need to sleep. It is said that they consist entirely of dead time, which they "kill" by lighting and smoking the cigars they make from dried hour-lilies. The implication is that the cigars are their sole sustenance. The Men are never shown eating or drinking, and presumably would not need to ever use a bathroom.
They Men in Grey are able to collect time more quickly than they spend it, since their lair includes a huge vault where millions of hour-lilies are stored. This implies the ability to move quickly from one human heart to another and "harvest" numerous hour-lilies in one clock-hour. (In an individual heart, only one hour-lily blosssoms per hour.)
The Men in Grey try to convince people to save time, which will open ever more human hearts to them, so that they can gather more time and create more Men in Grey. They seem to have computer-like mathematical skills, and may perform fabulous calculations (calculating entire lifetimes by the number of seconds) to convince a person that he or she is wasting time. However, the mental capabilities of the Men in Grey apparently do not include perfect memory, for they are sometimes described as taking down notes in grey notebooks.
When they leave a person they have talked to, the person normally forgets about their existence. Momo does not, apparently because she was able to resist the deal offered by the Man in Grey that visited her (he tried to make her play with a talking, Barbie-like doll demanding ever more things).
The Men almost seem like clones of one another, and plainly have no other choice than to serve the highly-disciplined organization that brought them into being. They have no names; each member is designated simply by an alphanumeric code (e,g,, "Agent No. XYQ/384/b"). Yet they are capable of some individuality. They sometimes debate among themselves how they should proceed, and then various viewpoints are presented (in a formal style of speech suggesting that the Men are the paranormal equivalent of deeply cynical technocrats).
They do care about their own existence as individuals, and when their supply of cigars is threatened, fierce fights may break out over the stolen time that remains. When organizational discipline is most keenly felt, as in their own lair, they are however ready to give up their individual life with no protest beyond a groan. This is seen when they deliberately decimate their numbers in the face of a crisis threatening their supply of time.
While often quite unfeeling, the Men in Grey are capable of some eagerness and passion, especially when they believe that triumph is near. However, at no point do Men in Grey express any grief whatsoever for comrades who perish. As one Man was condemned to be deprived of time for (inadvertently) having revealed certain secrets to Momo, he did scream for "mercy" when his cigar was taken from him, as if assuming that his own kind was not altogether beyond such a sentiment. However, if the Men are after all capable of mercy, none was shown in this case.
Momo was able to baffle one agent by asking him if there was really no one loving him, as if the very concept was alien and disturbing to the Man in Grey.