The Doctor pressed a series of keys on the TARDIS console and the monitor burst into life
"Come here Rachel"
Rachel approached the TARDIS console with a face full of anger
"What are you showing me?"
"This is an Events Extrapolation Unit or EEU. It monitors what we do and what the TARDIS does and can compute alternative paths. I am going to show you what in all likelihood would have happened had you killed Hitler"
Rachel watched the monitor, she saw herself pulling the trigger and Hitler falling to the ground with the bullet piercing his heart. The image faded away and was replaced by what seemed to be a Nazi propaganda film from 1973.
"What is this?" she asked the Doctor
"Hitler's death in 1923 made him a martyr. Many Germans who wouldn't had supported the Nazis rallied behind the party. Hess took over as leader and was able to use Hitler's death as a rallying call. Hess became Chancellor in 1933. He and Goering placed Germany on a war footing. Germany invaded Poland in 1939 and World War 2 began"
The screen showed a reconstruction of what appeared to be the fighting at Dunkirk. It showed allied soldiers fleeing into the English Channel. Then it showed a squadron of jet fighters swooping in from the sky and massacring them. The Doctor continued.
"in reality Goering was overseeing the upgrading of the Luftwaffe to using jet fuel technology. It didn't happen in our world but in this reality the support that Hess and the nazis got was translated into Deutschmarks and scientific knowledge. There was no evacuation of Dunkirk, no fleet of little ships, no finest hour"
The screen continued. It showed a graphic of the Nazi War Machine invading the UK. Images of London, Manchester,Sheffield, Edinburgh and Cardiff festooned with swastikas. It showed Edward VIII back on the throne with Wallis Simpson as Queen and Mosley as Prime Minister but they were puppets controlled from Berlin. King George's family escaped to Canada. Churchill was executed
The Nazis landed in the Irish Republic and invaded Ulster reuniting north and south. Ireland became an ally of Germany.
"With no-one to stop them the Nazis used bases in the UK to launch a full scale invasion of the USA. Within 6 weeks they took Washington. Roosevelt was killed when a bomb hit the White House...Your grandfather came from Chicago?"
Rachel nodded.
A picture showed a city in ruins
"That is Chicago after the Nazis razed it to the ground in 1940"
One final image appeared on screen. It was a German document but had an English name...Jeremiah Cassidy. It was his death certificate. The screen turned off.
Rachel started to sob. The Doctor took her by the waist and guided her back to the chairs.
"I'm sorry I had to do that Rachel but you had to know what would have happened. In history, our history Hitler's so called thousand year Reich lasted for 12 years from 1933 to 1945. One reason was that Hitler was deranged. You eliminate him and someone like Hess takes over, someone who was evil but not deranged. Someone who could have made the Reich last for a thousand years. Don't forget that it was Hitler who ordered the advance at Dunkirk to stop. Hess, Goering, Himmler could have ordered the massacre of the allied troops"
Rachel sat quietly with her head in her hands with a look of bewilderment and desolation on her face. The Doctor continued"
"Rachel, time is like a cliff-edge. Brittle, unstable and dangerous. One mis-step here or too much pressure there and the whole thing could collapse. Time travel isn't just about watching history unfold its about understanding its power and respecting its motives. I understand why you wanted to kill Hitler but the universe has a role for all of us and can't be changed not by a human and not even by a Timelord"
(From the 2002 Target Books Novelisation of "Munich" by Robert Sloman)