The protagonist is a low class demon who comes to an earth town to train. His motive is to store up human 'desire power'. It is a power that is released from humans when they have an appetite for worldly things, especially sexual lust. It's a useful energy for demons.
But instead of training and gathering energy from humans he somehow gets caught by the Mikuriya sisters and forced to do ecchi things with them. He can't gather desire power like this! Will he be able to train and become a higher level demon or...