What if at the outbreak of war in 1939, the Imperial Japanese Navy was on a world cruise and was in port in Kiel, Germany?
Japan still maintains a small skeleton fleet of ships in Southeast Asia, enough to run their empire and hold their territories. But the bulk is in Germany. So they can't conduct any meaningful offensive operations in the Pacific like Pearl Harbor.
Let's also say that returning all the way to Japan during wartime would be impossible to due hostile ports and Royal Navy activity.
So the Japanese navy's only meaningful option is to assist Nazi Germany, in the hope of eventually being able to return home by subduing Britain. They carry blueprints with them so that German ports and industries could manufacture spare parts and ammunition for the Japanese ships.
How would this play out? Would Operation Sea Lion succeed if Germany + Japan could rely on Japanese aircraft carriers for air superiority and troop transport?