Because

Keisuke has taken on the debt of a junior, and has borrowed money from the black-market lender, Suganuma. But, when the loan is due, Keisuke can't return all the money. So Suganuma asked to do H things with him in exchange for the interest due. While using his body to pay back the money due a relationship grows, Suganuma starts to be attracted to Keisuke's bravery.....
Will the situation go on as expected?!