The anime is set in an alternate version of Heian-kyou, the old name of Kyoto and the capital during Japan's Heian era. This alternate version of the city is named Denji Heian-kyou. A delinquent high schooler named Takeru Narehira resides in our world, but after an unfortunate accident, is transported to Denji Heian-kyou.
 
Here, he immediately meets Tsukimiya, a woman he has seen many times before in his dreams. Hardly a moment passes after that encounter when Takeru and Tsukimiya encounter an oni monster, and lose their lives.
 
Takeru awakens, stunned to find himself again at Denji Heian-kyou. He puts together that he has the power to go back in time. Now he resolves to learn this world's version of mystical onmyōjutsu techniques, and become an onmyōji sorcerer to save Tsukimiya. 
(Source: Anime News Network)

Onmyo Kaiten Re:Birth Verse