Joh is the father of Freder Frederson. He is the mastermind of Metropolis. After meeting with his shaky ally, Rotwang, the mad scientist, he is taken to the catacombs of his city, and sees Maria delivering the story of the Tower of Babel.
Joh considers her a threat to his power, and orders Rotwang to remake The Machine Man in her image in order to fool the workers into bending to his will. He will execute this plan by having the false Maria deliver pro-upper class speeches to the workers.
At the end of the epic story, Joh witnesses his son, Freder, engaged in fisticuffs with Rotwang, and when Freder wins the duel and comes down to him, Joh is grateful to him.
Freder has his father shake hands with the leader of the workers, therefore fulfilling the prophecy of him being the "Mediator".