Okay, so you know how sometimes people live in different places and have different cultures and ways of doing things? And sometimes they feel really proud of where they come from and call it their "homeland" or "motherland"?
Well, in the movie Fatherland, it takes place in a place called Nazi Germany, which was a long time ago when some really bad things happened. The people in this place felt really proud of it and called it their "fatherland."
The story is about a man named Xavier March who is a detective and is trying to solve a mystery about a dead person. But as he investigates, he starts to uncover some really big secrets that the leaders of his country have been hiding. These secrets are about some terrible things that had happened in the past, like a war that killed a lot of people, and how the leaders had been keeping it all a secret.
So Xavier has to decide what to do with this information, because it could change everything that people thought about their "fatherland." And that's what the movie is all about - learning about history and deciding what to do with the truth.