After being kidnapped by the cult Gorgom in the night of their 19th birthday, step-brothers Kotaro Minami and Nobuhiko Akizuki were submitted to a mutant surgery with the purpose of becoming the candidates for the next Gorgom's Creation King. Kotaro escaped before the brainwashing, with help from his foster father, and turned against Gorgom. He soon finds out the horrific truth from his step-father: Gorgom originally killed his true parents and, since both Kotaro and Nobuhiko were born on the day of a solar eclipse, referred as "Black Sun" and "Shadow Moon". Kotaro, taking on the name of Kamen Rider Black, was determined to rescue his step-brother from Gorgom while protecting Japan. However, later in the series, Nobuhiko emerged as Shadow Moon to fight Kotaro with the survivor becoming the next Creation King.