how does he get involved?

I'm planning a sequel to one of my stories. The first story involves a Chessmaster kid with strange powers MindControlling a vampire detective/slayer-killer into being his minion. The second story takes place ten years later, when the vampire detective has mostly recovered from the effects of the Mind Control (it's not an instant-recovery sort of thing, he had to systematically challenge each idea the guy planted in his head).

He's taking on his first case since the Mind Control, and doing better than he expected. But it turns out the same guy who mind controlled him before is back, and has been subtly planting thoughts in his head using a 'backdoor' he made back then. He's helping the guys the vampire is after, and leading the vampire astray.

But I can't think of how he'd get involved. The events of the first book are secret, so the target of the investigation couldn't just seek him out. And the kid is very self-serving and pragmatic, so he won't just help the slayer because he thinks it's a good thing to do. He'll need to personally benefit somehow.

What is the target organisation after? And the kid's main motivation?