In the alternative world in which The Plot Against America is set, FDR is defeated in the 1940 presidential election by Charles Lindbergh, the famous aviator, isolationist, and fascist sympathizer. The new Lindbergh Administration signs treaties of understanding with Nazi Germany and Imperial Japan, allowing the Axis powers a free hand in their respective spheres of influence, as well as keeping the United States out of the war.
On the domestic front, life becomes harder for Jewish citizens, as they are forcibly relocated to rural areas, where it is hoped that they will become "better Americans." To make matters worse, Jews are regularly subjected to violent pogroms, eerily similar to those that took place in Nazi Germany during Kristallnacht.
One day, however, Lindbergh suddenly vanishes. In the ensuing chaos, an emergency election is called which is subsequently won by FDR. The United States enters the war on the side of the Allies and the normal trajectory of events is re-established.
The twist of the story—spoiler alert!—is that President Lindbergh's baby, long thought to have been kidnapped and murdered years earlier, is actually being held hostage by the Nazi regime to give itself leverage over the President. In other words, the Nazis have essentially been blackmailing Lindbergh into doing their bidding all this time. A hostile foreign government has successfully used its leverage over the right-wing celebrity President to interfere in American domestic politics, severely undermining the values of American liberty and democracy in the process. Thankfully, this is all just a fantasy, and would never actually happen in real life.