Operation Sea Lion was Hitler's favorite war plan, the invasion of Britain across the English Channel. On July 16, 1940, Hitler ordered the preparation of a plan for the invasion of Great Britain. An amphibious invasion would only be possible if Germany could establish control of the air in the battle zone, due to Britain's large navy. To this end, the head of the Luftwaffe, Hermann Göring, on August 2 issued the "Eagle Day" directive, laying down a plan of attack in which a few massive blows from the air were to destroy British air power and so open the way for the amphibious invasion, termed Operation Sea Lion. Victory in the air battle for the Luftwaffe would indeed have exposed Great Britain to invasion and occupation.
The plan indicated the German beachhead to be located around Dover, where the panzers would lead the charge into London. The hypothetical German D-Day was very similar in tactics to the Allied D-Day in 1944. Once London was captured, the Germans expected British resistance to crumble. In reality, that assumption was open to serious debate, given the tenacity of the British during the Battle of Britain and the morale of the nation embodied by Winston Churchill.
Thankfully, this operation was never executed because the RAF successfully fended off the Luftwaffe in the Battle of Britain, which discouraged Hitler from invading Britain and turned his attention to the Eastern Front. Given that America was not yet actively involved in the war yet and the Waffen-SS (the regular German army/infantry) outnumbered the British army, if the Germans were allowed to invade the odds looked very dim for the British. After World War II ended, documents exposed the plan the Nazis had in store for the British had they been defeated, and it involved extermination camps. Hitler never liked the Brits, apparently, unless they were in a position to give him more power or they were visiting dignitaries, not least of which was Charles Lindbergh, recently-abdicated English King Edward VIII and his wife Wallis Simpson.
[Ed. note: fixed a definite article problem 11/4/2001.]