Once upon a time in Okayama, Japan, lived an old man and an old woman who had no children of their own. One day, the old man went into the forest to cut down bamboo while the old woman went to the river to wash clothes. While at the river, she noticed a giant peach bobbing up and down in the water. She took the peach from the river and brought it home to show to her husband. In the evening, when the husband came home, he started to cut open the peach to eat it, but before he could, the peach split open by itself and a baby boy jumped out. They named him Momtaro, the Peach Boy. So far so good, eh?
But one day years later, a marauding demon (oni) came into Momotaro's village and stole things from the people. The demon came back often, especially at night when the villagers were sleeping, and caused havoc. Momotaro, now a brave, strong boy of 15 years, decided to fight the demon to save his village from its wrath. Now, you don't actually believe this part, do you?
I'd say that the "demon" was likely an outsider who had moved to the village recently and didn't know anyone. Maybe he worked for a woodcutting company who transferred him to a new bamboo forest branch. But due to the villagers' paranoia of outsiders, they blamed the newcomer for every petty theft or lightening strike that befell the village.