In the ruins of an
amphitheatre just outside an unnamed city lives Momo, a little girl of mysterious origin. When asked, she replies, "As far as I remember, I've always been around." She is remarkable in the neighbourhood because she has the extraordinary ability to listen. By simply being with people and listening to them, she can help them find answers to their problems, make up with each other, and think of fun games. The advice given to people "go and see Momo!" has become a household phrase and Momo makes many friends, especially an honest, silent street-cleaner, Beppo, and a poetic, extroverted tour guide, Gigi (Guido in some translations). This pleasant atmosphere is spoiled by the arrival of the Men in Grey, eventually revealed as a species of paranormal parasites stealing the time of humans. These grey-clad, grey-skinned bald men present themselves as representatives of the Timesavings Bank and promote the idea of "timesaving" among the population: supposedly, time can be deposited in the Bank and returned to the client later with
interest. After encountering the Men in Grey, people are made to forget all about them, but not about the resolution to save as much time as possible for later use. Gradually, the sinister influence of the Men in Grey affects the whole city: life becomes sterile, devoid of all things considered time-wasting, like social activities, recreation, art, imagination, or sleeping. Buildings and clothing are made exactly the same for everyone, and the rhythms of life become hectic. In reality, the more time people save, the less they have; the time they save is actually lost to them, consumed by the Men in Grey in the form of cigars made from the dried petals of the hour-
lilies that represent time. Without these cigars, the Men in Grey cannot exist. Momo, however, is a wrench in the plans of the Men in Grey, thanks to her special personality. Various plans to deal with her directly fail, so the Men in Grey start targeting her friends instead. When everyone else has fallen under the influence of the Men in Grey in one way or another, Momo meets Cassiopeia, a
tortoise who can communicate through writing on her shell and can see thirty minutes into the
future. Cassiopeia leads Momo to the administrator of Time, Master Secundus Minutius Hora, where they form a plan to defeat the Men in Grey. Hora stops time, freezing everyone except the Men in Grey, Cassiopeia and Momo, thanks to one last hour-lily he has given her. With their supply flow cut off, all the Men in Grey rush to their underground lair, where Momo follows. She observes as they decimate their own number in order to stretch their stockpile of time as far as possible. With the advice of Cassiopeia and by using the hour-lily, Momo is able to shut the door to the vault where the stolen lilies are kept. Now facing extinction as soon as their cigars are consumed, the few remaining Men in Grey pursue Momo, perishing one by one. The last Man in Grey finally begs her to give him the hour-lily so that he can open the vault. When she refuses, he too vanishes, remarking that "it is good it is over". Using the last minute she has before her hour-lily crumbles, Momo opens the vault again, releasing the millions of hour-lilies stored within. The stolen time returns to its proper owners, causing time to start again (without people knowing it had ever halted). Momo is reunited with her friends, while Master Hora rejoices together with Cassiopeia. ==Major themes==