From wiki:
The story revolves around "Enchanters" or Kikō majutsushi ("machinist-magicians"), people who can build enchanted items with the power gained from their contracted demon.
The enchanted items can then carry special effects like "hardened", "water resistance", or effects like "evil repelling".
The protagonist, Haruhiko Kanou, is a regular high school student with the gift of fixing and building mechanical and electrical appliances. His motivation comes from his secret love towards his next door neighbor and childhood friend, Yuka Fujikawa, who is a few years older and now teaches at Haruhiko's high school.

An enchanter, Fulcanelli, and his beloved contract demon Eukanaria roams the secretive world of magic and sorcery with Fulcanelli being the top. Fulcanelli, however, died a little before the start of the story, and Eukanaria wanted to revive him with the stone that contains his soul. But Fulcanelli had no wish to be revived, but didn't want to hurt Eukanaria and kept silent.

Eukanaria found the perfect body for Fulcanelli, which is the protagonist Haruhiko. To Haruhiko's surprise, Eukanaria looks exactly like Yuka, and he himself looks exactly like Fulcanelli. Eukanaria tried to seduce him into giving her his soul so she can place Fulcanelli's soul into Haruhiko's body using four different procedures. Haruhiko either has to sign a contract, die, have sex (there is a moment when the soul becomes unstable) or when he loses the will to live.