Two thousand years ago, the stone sword known as Sekiken was involved in a tragedy that plunged the land of Kamitsumihara into chaos. Now, in the present, all the elements needed to perform a ritual to cleanse the land and end the endless cycle of reincarnation are available. Kamiazuka Touka, current holder of the Sekiken, meets Kawakabe Momoka, a young girl and reincarnate of the tragedy. During their brief time together, they must uncover the truth behind past events, try to lead a normal life, and learn what it’s like to be human.
Hatsumi and Hazuki are two best friends who share a bond like sisters, and perhaps something more. With newly found (and forbidden by nature) feelings of romance developing for Hatsumi, Hazuki finds her world becoming more and more disarrayed. She finally works up the nerve to confess her love shortly before Hatsuki's 16th birthday, but suddenly, Hatsuki disappears in a green light, leaving no trace. Now, with the help of the mysterious Lilith and a feathered friend, Hazumi sets out on a journey through a plethora of worlds to find her love and bring her back safely.
Well, first off, the two shows originate from visual novels made by the same company (ROOT). They also share the same director (Yuji Yamaguchi), music composer (Tada Akifumi), animation studio (Studio Deen) and character designer. Plus, the main characters from YamiBou return in Touka Gettan for a single episode, just enough to give you a glimpse of the world they live in. Both shows also use shoujo-ai as their main theme (and hints of incest, though it doesn't necessarily go far). They're both very underrated shows that most (if not all) shoujo-ai fan will love.