He's bored one night, randomly clicking through website after website of girls putting themselves on display. All out porn isn't doing it for him anymore, so when he stumbles across a link to a cam site he's a little more interested. One girl catches his eye, for no reason other than that she's wearing a uniform skirt that tugs something in the back of his mind. He watches for a bit, distracted, but he's nearly got himself off when her face dips back into frame and he realizes that not only does he know her well, she's also very much underage.
She can be whoever, as long as it's taboo: his student, niece, patient, boss's or friend's daughter, employee, etc, I'd just really like something where he can use this knowledge to blackmail her into becoming what he wants. I'd like to combine a dark, kinky line with actual care and attachment that builds between them to where he realizes that he needs her even more than she needs him, not just straight up smut all the time.