Found it. It's the second story in the series. The series started when NCIS is investigating a murder. The victim had apparently been stalking his neighbor- was obsessed- stole his stuff and videotaped them in bed. They were hardcore D/s. After accusing the neighbor's partner, Tony wind up clearing them both. He likes them (as people) and arranges to get all their stuff returned to them (including the videos)That's where this story, Right Angles (http://blueraccoon.livejournal.com/469233.html#cutid1), picks up. He's interested in D/s somewhat and near the end they explain some.