Tim Daly is set to guest star in the two-hour episode of ABC's Grey's Anatomy.

He's designed to serve as a springboard for a potential spinoff from the hit medical drama, The Hollywood Reorter. Paul Adelstein also has been cast as a guest star in that episode with an eye to co-star on the potential spinoff series toplined by Kate Walsh.

The story line for the two-hour episode is being kept under wraps, but it is understood that it will feature Addison Montgomery-Shepherd (Walsh) on the verge of leaving Seattle Grace, a scenario that will be triggered if ABC goes with the spinoff for next season. Daly's character is described as "handsome, sincere, like a McDreamy." It was not clear what character Adelstein will play; the breakdown also includes a female psychiatrist and a female fertility specialist.

Grey's Anatomy airs Thursday nights on ABC.