'Grey's Anatomy': Addison and Derek get back together! Sort of.

Calm down, Mer-Der fans. This is not the apocalypse.

Yes, "Private Practice" star Kate Walsh is returning to "Grey's Anatomy" for another crossover event, and yes, in this episode, Addison Montgomery will be with Derek (Patrick Dempsey) once again. In the romantic sense.

However, there's a twist. TVLine reports that the episode, set to air in February, is a look at what would have happened at Seattle Grace if Meredith's mother, Ellis Grey, never had Alzheimer's. The idea is that if her mother had continued on as a world-class surgeon, Meredith (Ellen Pompeo), would be a very different woman today.

In the episode, Walsh returns to play Addison, who -- yep -- never broke up with McDreamy. They're still married; we're just not too sure it's a happy marriage. We can also expect the return of other former cast members, including one who died in the hospital shooting back in Season 6.

"Grey's Anatomy" is back on Thurs, Jan. 5 at 9 p.m. on ABC. In the meantime, we're still mourning Henry (Scott Foley). Why, cruel world? Why?!