Get ready for Addison's return ... again!

Private Practice star Kate Walsh is heading back to Grey’s Anatomy in early January to play a key role in a top-secret crossover involving Addison, Mark, and guest star Leven Rambin.

The situation as we know it is as follows:

In the Grey’s Anatomy half of the crossover, Mark Sloan (Eric Dane) summons Addison to Seattle Grace to perform surgery on Rambin's character, who was announced last week.

The storyline spills into Private Practice when, according to creator Shonda Rhimes, “complications arise and Mark ends up taking [her] back down to L.A. for more surgery.”

As for Mark’s connection to this new crossover character?

That remains to be seen. As does whether sparks fly between lovers-turned-friends Addison and Mark, and what impact this new crisis has, if any, on the future of Mark and Lexie.

Answers to all those questions will likely be revealed, or at least hinted at, in future spoilers.

For now, it's up to us to debate them right here. What do you think happens, and are you excited for the crossover? Sound off on this Grey's Anatomy-Private Practice news below!

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.