The world, lying in ruins, pollution and surrounded by war, faces unclear times of a lot of death and diseases. Doctor Azuma has found a solution to regenerate human bodies and so help the humanity. However, he is not able to complete his work and the humanoid creatures which calls themselves 'the new humans' went out free to destroy the current human population and create their own world.
Both discuss dark themes about society and science in a sci-fi environment. While Casshern is darker and features more action, metropolis tries to approach the topic in a more colorful way, followed by jazz music. You might want to check the two different approaches to this particular idea.