Hitler is captured in Berlin by the Russians, 1945; surprisingly, they don't immediately shoot him (or Eva Braun) because Joseph Stalin has ordered that if Hitler is found he is to be taken alive at all costs, so he can be put on trial. So Hitler's trial begins in Moscow, 1946; Stalin's intention is to embarrass the West by pointing out that there were a lot of people in the West who were sympathetic to the fascist ideology because of their fear of Communism.
In exchange for his life, Hitler agrees to testify about links between the Nazi Party and fascist parties in every other country in Europe--including the Soviet Union and its satellites, the members of which groups Stalin wants to track down and annihilate.
Hitler, OTOH, wants to use the trial as a chance for a rant for Fascism/against Communism; although his attempts to do so are blocked at the trial, he does manage by various means to write a book in prison, and to get it smuggled out of the USSR. It reveals the full extent of the Nazi/Soviet agreements in 1939, and paints Stalin in such a bad light that he is immediately deposed and executed.