Okay, imagine if during World War II, instead of winning the war, the bad guys (called Nazis) actually won and took over a large part of the world including the United States. So now, it's like America is not America anymore, it's something called the "Greater Nazi Reich".
In this world, there's a book called "The Grasshopper Lies Heavy" that tells a story about how things could've been different if the other side (the good guys, called Allies) had won the war instead. And people are really fascinated by this book because it's like a glimpse into a different world.
One lady, Juliana, gets her hands on this book and she becomes really curious about it. She starts to look into it more and try to figure out why it's so important.
At the same time, there's this other guy named Joe who works for the Nazis but he doesn't really believe in what they're doing. He's kind of a rebel and starts to realize that maybe things aren't as great as they seem in this world.
So basically, "The Man in the High Castle" is about these different characters trying to figure out what's going on in this alternate world where the bad guys won the war. They have to be careful because if anyone finds out what they're doing, they could be in big trouble. But they also want to know the truth and try to figure out if there's any hope for things to be different.