In what is being billed as a major Grey's Anatomy "event," a massive tragedy hits Seattle Grace as an ambulance crashes and threatens the lives of not only the patient(s) inside, but the lives of the doctors and paramedics themselves.

The Chief and Dr. Bailey work tirelessly at the crash site trying to saves the lives of both the patients and the paramedics... although they encounter at least one man (see sneak previews) who refuses their treatment on controversial grounds.

Seth Green also guest stars as a character named Nick.

But that's just the beginning of the drama that begins tomorrow, on Thanksgiving night, in the first half of a two-part episode, entitled "Crash Into Me, Part I."

We've gotten to know more about Dr. Alex Karev (Justin Chambers) at the end of last season and the beginning of this one. Now it looks like he reaches a turning point when Ava shows up (again) and meets the new girl in his life - Lexie Grey.

Is a new Grey's Anatomy love triangle developing between Dr. Alex Karev, Ava / Jane Doe / Rebecca Pope (Elizabeth Reaser, right) and Dr. Lexie Grey (Chyler Leigh)?

As always, you can discuss this episode and all things Grey's Anatomy by leaving comments or talking about it with fans in the Insider's exclusive FORUM.

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