After Mazinger Z and Aphrodite A defeat the Mechanical Beasts: Zaurus F1 , Buragga S1, and Demonger J5, the female demon Sirene is released from underground and leaves without anyone but Dr. Hell noticing a thing. At the Himalayas, Dr. Hell frees the other demons including the demon general Zannin as part of an agreement for both sides to take down their enemies (Mazinger Z for Dr. Hell and Devilman for the demons). Devilman had spied on this conversation, and goes to warn Koji but ended up mocking the Mazinger Z for not being able to fly which lead to the hot-blooded teens engaging in a motorcycle race that ended up a draw. The Photon Power Laboratory had completed a way for the Mazinger to get over its disadvantage against flying enemies, the Jet Scrander. A demon was spying and reported back to Dr. Hell who sent a shapeless demon named Bugo and Sirene to destroy the Scrander. While they manage to damage it, they are driven away by Koji's raygun with Sirene taking Shiro and Sayaka hostage. She drops them when reaching a certain height but they are saved by Devilman (Koji learned Akira's secret when coming to rescue them). Koji returns the favor to Devilman by having the Mazinger save him from the amorphous demon in the sea and they join forces to fight against the forces attacking the Photon Lab but Devilman is captured and tortured. The Mazinger comes to the rescue with a repaired Jet Scrander before killing the demons and destroying Hell's flying fortress with Hell and Baron Ashura retreating. With Koji and Devilman on good terms, they fly off into the sunset.