I have to say that I was a little confused for the majority of this episode. "When was the Grave Digger mentioned?" "When did they get kidnapped?" "What the hell?" all were running through my mind. Twice, I paused the episode to run through Sidereel's catalog of the last season and a half's Bones summaries, wondering if I'd just forgotten something. Several times I wondered to myself if maybe the good folks at Fox hadn't screwed the pooch and aired an episode out of order.
Finally, I read someone else's synopsis of this episode, and was rewarded with some info. Yes, the Grave Digger had been mentioned before... in SEASON 2! Apparantly I missed that episode. Season 2, episode 9, to be exact, "The Aliens in the Spaceship". So I just went and watched that one (great episode, BTW). Okay, much more in perspective now. But... really? A reference to a killer from 2 and a half seasons ago? That's asking a lot of the viewers, people. That's asking us to not only have seen it, but to remember. Yes, you can still get a lot out of the main plot device (Booth's kidnapping) without having seen it, but you miss so much about the underlying drama and who some of the characters are, and from where their motivations stem.
So, Bones producers... 2.5/5 stars simply because you relied too much on an experience from 2.5 freaking season ago! Bones fans...if you've seen and remember S2E9, then great, view the new one. Otherwise, go back and watch it first. Trust me, you'll thank me.