The series tells the story of the battle between the Earth’s defenders and a demonic entity named Genma Daioh. Its first protagonist is Jo Azuma, a Japanese highschool student of the 1970s who suddenly finds out he has psychic powers and is being called along with other psychics around the globe. They are gathered by Vega, an alien cyborg warrior, and Luna, the princess of Transylvania, who have discovered the presence of Genma traveling towards the Earth with the intention to conquer it. In order to stop him, Luna and Vega train Azuma and all the other psychics to defeat Genma and thwart his destructive plans. However, despite initial victories, the manga ends in an ambiguous cliffhanger, hinting that the psychics are defeated and the Earth will be destroyed.
After the premature end of the manga, the story was expanded upon by other series of the franchise, featuring time travel and battles in other realities. This is first shown in the sequel New Genma Wars, which takes place in 1999, after Genma’s victory has turned the Earth into a post-apocalyptic wasteland ruled by the demon. In that place, a surviving psychic named Chinami Kagawa is entasked by Beatrice, princess Luna’s daughter, to travel back to the Edo period and create a new clan of psychics who could defeat Genma. She is opposed there by Yui Shōsetsu, who has fused with an incarnation of Genma come to chase Kagawa. (This series also ties with a previously unrelated Hirai manga, Adult Wolf Guy.)
The story is taken then to True Genma Wars, after Kagawa’s exploits rewrote reality. In a new version of 1979, Jo Azuma, now an adult paranormal researcher and writer, is hired by a powerful woman known as Moonlight to expose the existence of psychics to the world. After several events, Jo gains the knowledge of the previous timeline destroyed by Genma, only to disappear mysteriously. His mantle is taken by his secretary Yuri Sugimura, who travels to the 7th century to meet the mystic En no Gyōja. Also, in another time plane, an ancient Greek girl named Chronos gets involved in a war against Mu, an ancient civilization populated by mysterious beasts who might be related to Genma.
The next installment is Genma Wars: Eve of Mythology Chapter, though it takes place again in the original timeline, where the world was conquered by Genma. Humans now live in primitive societies, while the planet is ruled by a race of intelligent Genma beasts serving Maoh King, who now plans to give birth to an heir by raping a human woman. However, said woman, Non, manages to run away with the resultant children, two male twins named Loof and Jin, who swear to stop their father. After defeating Maoh, they travel to the past in an attempt to prevent the existence of their terrible world, but agents of Genma from the past spoil their plan by provoking a nuclear war in substitution of the original Genma victory. Nonetheless, Loof and Jin take over the new timeline, ensuring a better future for mankind there.
A Hirai novel of 1979 similarly named Genma Wars takes a new route from New Genma Wars. being set on a timeline created there. A young version of Jo discovers in 1967 he has psychic powers which predict the coming of Genma, so he initiates a resistance movement from his school’s literature club, named “Genken”. As in the previous story, Joe disappears mysteriously, and the members of Genken are left to advance by themselves. In a sequel named Harmageddon, a member named Keisuke Takatori gives birth to a totalitary global movement based on his messiah complex.
Harmageddon Girl is a side story of Genma Wars featuring Michiko, Jo Azuma’s sister. She uncovers her own powers to save herself and a plane full of passengers from an attack by Genma, but is sent to ancient Greece as a result. Here, she meets Chronos, who leads her to the space in order to gather with several other characters of the franchise, among them Luna. At the end of the story, they get ready to battle Genma in his invasion of Earth.
The entire franchise is revisited in a 2014 manga named Genma Wars Rebirth and written by Kyoichi Nanatsuki. In it, Luna and Stella (Luna’s daughter with Jo) travel to the past in order to try to prevent the war.