'Grey's Anatomy' and its spin-off, 'Private Practice,' are at it again with their crossover episodes.

The Ausiello Files is reporting that Walsh will come back to 'Grey's' in early January for a top-secret plot centered around Addison, Mark and Leven Rambin, who recently joined the cast from 'Terminator: Sarah Connor Chronicles.'

Apparently, Addison returns to Seattle at Mark's request to perform surgery on Leven's character. But, as executive producer Shondra Rhimes spoils, the plot will extend back over into 'Private,' when a complication has Mark transfer the patient back down to L.A. again (so much for the plot being "top secret").