As a sadomasochist, Sade just wants to get and give pain in manageable and somewhat legal doses.
All Mercy wants is to walk in her father's footsteps--a man who sacrificed everything to save her from a life of abuse and suffering.
When Mercy finds an envelope on her doorstep leading her to Sade, the result is a collision of pain and mercy.
For Sade, Mercy's kindness is dessert for his sadism, and her strength has his masochism hot and begging. Exploiting both for his own pleasure is his plan until he realizes that Mercy's got her own game. And playing it requires something he's never had.