Ume Soft - Do M Otoko Tantei ga Iku Katte ni Ittara Oshioki yo!

The story so far: Our pathetic, lazy hero and his childhood friend Satomi, as beautiful as she is brilliant. He lives his days in a hardboiled fantasy, playing at being a detective while barely managing to make ends meet. She's attending a top university and looking after him by acting as his reluctant partner.
Never quite managing to make it past "just friends", the two get wrapped up in a startling incident, and suddenly things are turned upside down! Due to the actions of a mysterious man, Satomi is transformed into a super sadist whose thirst for punishment cannot be quenched!

The sadistic Satomi toys with our hero, twisting him this way and that...perhaps finally going so far as to force him to claim her virginity!

"If I don't change Satomi back to normal quickly...I'll never be able to resist her punishment..."

What fate awaits our hero? Will he be able to track down the mysterious man and save Satomi? And what will become of the two of them?!