Lucy Monostone was a nobody musician being used by the devil herself.
The devil has the power to jump from body to body creating new personalities in the bodies she steals, and one of those stolen bodies was the daughter of Lucy Monostone who was the real mastermind behind everything, also she has been living stealing bodies since the beginning of time.
Tetora and Miwa Isono, were also the offspring of Lucy Monostone, but also of the devil, but also they are the experimental products of people trying to recreate the bodysnatching powers of the devil, so they are not really blood related, but more like siblings by mental test tube experiments.
So they are all have Monostone as their last name.
The devil wants to destroy the world and mind control everyone, because yes.
And right now Tetora Monostone and his sister/lover/childhood friend/something Miwa Isono Monostone are inside the mind of their own sister/mother/grandmother/ancestor/Lucy Monostone/The Devil Herself/Lucy's Daugther Machi Monostone having a mental battle in which everything they can imagine can become real.
In resume Tetora and Miwa are inside the head of Machi, who is being controlled by the devil herself, and the devil want's to destroy the world. Oh and Lucy Monostone has a son or a grandson, and he is supposed to do something, but he hasn't done shit, oh and since Lucy was controlled by the devil, that means that his father or grandfather was the devil herself, so his dad is his mom, but this time it is special because this time sex happened, that means he is the biological descendant of Lucy and not just a clone or mental clone like everyone else.
So in this mind battle in order to exorcise the devil out of Machi and save the world, Tetora and Miwa fused to create Mitora Monostone (who the devil thinks is the realest Lucy Monostone, despite she being behind everything Lucy ever did) and pilot him like a Jaeger from Pacific Rim in order to kill a horde of zombies, while a giant eye ball chases them, so that they can get into a cellphone tower and force out the devil herself out of Machi (to who knows where, since the devil doesn't has a body, she just goes from mind to mind).