Massive Multiplayer Crossover
Pandora, of course, is the Bride of Frankenstein.
Cesare from The Cabinet of Dr. Caligari is Dr. Pretorius's assistant and valet instead — and it turns out he's a pretty nice young man when he's not being brainwashed into committing murders while sleepwalking.
Henry Frankenstein presses Hans Beckert, the whistling child-killer from M, into his service.
Dr. Rotwang builds his robot girl on the outskirts of Berlin in the 1920s rather than in the fantastical future of Metropolis.
And Lulu from Pandora's Box performs at a club with his robotic creation.
Christopher Isherwood (himself a Historical Domain Character) offers to introduce Pandora to a friend of his who performs at another club, an American singer.