The orphan girl Momo arrives at the ruined amphitheatre on the outskirts of the city and decides to live there. She quickly makes friends, who feel their lives so much improved because of her friendship and company.
Momo and Beppo the street sweeper listen to Guido the storyteller make up a fairytale.
Niccóla the hairdresser is approached by an agent of the Timesavings Bank ...
... and is persuaded to open an account with them.
The Men in Grey do their nefarious business, stealing time from more and more people in the city.
Momo meets Lola the Living Doll, one of the many fancy new toys the children in the city are given to keep them busy when their parents have no time for them.
But Momo isn't impressed.
One of the Men in Grey shows up at the amphitheatre to try to get to Momo ... but instead it's she who gets to him, and he spills all the Bank's secrets.
In the meantime, everyone in the city is changing more and more. The children only care about their new toys.
Nothing Momo tries can stop what's happening ... but then Cassiopeia the turtle turns up, and takes Momo with her along the strangest paths.
A tribunal at the Bank decrees the worst possible fate for the agent who betrayed its secrets.
Cassiopeia takes Momo to the Nowhere House ...
... while the Men in Grey search for her at the amphitheatre and hatch an evil scheme to stop her from destroying all their plans.
Momo meets Master Time ...
... who shows her the Hour Flowers, the source of all human time.
Back in the city, Guido has become a celebrity like he always dreamed.
But he is completely at the mercy of the Men in Grey.
Beppo has searched everywhere for Momo and in desperation he takes his story to the authorities.
But they say he's crazy and against his will he's placed in an asylum.
Finally, Momo returns home. But all her friends are gone.
She tries to see Nino at his successful restaurant, but he's far too busy to have time for her.
She manages to find Guido, but even he is too busy, with all his new commitments.
Momo is all alone and desperately lonely. She has even lost Cassiopeia.
The Men in Grey see their chance, and approach her with an offer they think she can't refuse.
But she will never give in to them. Cassiopeia returns and Momo decides that they have to go back to Master Time for help.
The Men in Grey try to follow, but they can never enter the Nowhere House. They lay siege to it instead.
Master Time reveals the entire conspiracy to Momo and Cassiopeia.
She knows of a way the Men in Grey can be stopped, but she can't do it herself - she needs help from someone who can move freely in the outside world. Will Momo help? Yes!!
She takes the battle to the Men in Grey ... !
And is victorious!! The time thieves are defeated!
All the stolen time is released back into the world.
Momo is finally reunited with Beppo and Guido and all her other friends. Things go back to the way they should be and everyone in the city is happy again. :-)