Akira Fudo is a shy teenager who lives in his friend Miki Makimura's house as his parents work abroad. One day, Akira's childhood friend Ryo Asuka reveals to him that the Earth is about to be invaded by demons, monstrous beings hibernating for centuries in the ice who are about to resurface. According to Ryo, demons have lived on Earth before the appearance of man, and now they want to claim possession of it. The only way to defeat them is to take control of the demons' powers to fight them on equal terms. Ryo then involves his friend in a ritual called the Black Sabbath, an event where numerous demons plan to merge with humans to infiltrate society. During the Black Sabbath, Akira merges with Amon, a demon warrior both idolized and feared among his kind for his incredible strength. However, instead of Amon holding control over Akira, the latter's pure soul triumphed over that of Amon, bringing the demon to heel and creating Devilman.
After fighting out of the Black Sabbath, Akira uses his demon persona to battle multiple enemies hidden in society. However, Akira starts to question his methods after he encounters Sirene and Kaim, two demons whose relationship with one another challenged Akira's prior perception of all demons being immoral, and the cruel Jinmen, a turtle-like demon who contained the still-living souls of his human victims on his shell, forcing Akira to destroy them to defeat the demon. A large detachment of demons led by the demon Lord Zennon invades Tokyo, alerting the population of the world to the existence of demons. Akira learns that other humans are becoming Devilmen and resolves to contact them to form a team dedicated to protecting humankind from the demons.
Meanwhile, Ryo journeys back to the mansion where he informed Akira of the world of the demons and finds an album about his life which states he died years ago in a car accident. During his ensuing existential crisis, Ryo is met by a group of demons led by the demoness Psycho Jenny, who informs him of the reality of his identity. Ryo is Satan, a fallen angel who sympathized with the demons and turned on God for wanting to exterminate them. As part of Satan's plan to lead an all-out war on humanity, he had Psycho Jenny erase his memories and replace them with those of Ryo Asuka until the time when he was ready to begin the war. During a television broadcast, Ryo reveals the existence of Devilmen to the public but does not differentiate them from demons when he shows footage of Akira's first transformation. This leads to a panic that causes humans to turn on each other worldwide. While Akira confronts Ryo for answers and learns the truth of his former friend's identity, a mob of humans kills Miki and her family for being associated with him. With Miki's death, Akira loses faith in humanity before vowing to get revenge on Satan. When asked by Zennon about why he wanted Akira to survive humankind's extinction by becoming a Devilman, Satan implies that the reason is because he loved Akira and wanted to protect him. Twenty years pass, and all humans, save for the Devilmen recruited by Akira, are extinct. During their final battle, the Devilmen are wiped out by the demons and Satan kills Akira. Reflecting on his actions, Satan realizes what he did to humankind was no different from what God did to demons, and as he feels regret, God's angels descend upon the Earth.
