We cut to big baby Derek blowing off Addison, again, and her calling him on his Meredith hang-up, loudly…at work, of course. In front of everybody, even random patients. Privately, in the stairwell, she tells Webber, "The only people who don't know Derek loves Meredith are Derek and Meredith. How do I compete with that?" Let's ask a People editor, shall we? And here's one to tell us, "I think that she realizes during this time that it's the beginning of the end." Cut to Addison and Derek, post-surgery. "We're trying," she says hopefully, and Derek agrees. Then he says, "Addison, will you go to the prom with me?" And I think, Wow, the characters are actually acknowledging that they're acting like melodramatic teenagers! That's a pretty funny joke. Except...it's not a joke, because apparently there was an actual prom? At the hospital? Sigh. So Derek and Meredith gaze dreamily at each other over the shoulders of their respective slow-dance partners (barf), and Derek excuses himself to go make out with Meredith. A subtle shot of the firmly erect Space Needle takes us to their lovemaking montage, in a supply closet or wherever they are. Hell, this is Seattle Grace; they're probably in the middle of the ER waiting room. After a few sexy moments, we see Mer freaking out about her missing black panties, and Derek getting all mopey, asking, "What does this mean?" Uh, briefly, it means you're a jerk. (I didn't even need a People editor to explain that!) The scene ends when Callie walks in on them getting dressed -- although Callie's discovery will not be referenced at all in this special, so it's an odd detail to include. It's almost like this promo wasn't edited all that carefully. ...But that can't be, can it?
Now Julie Jordan is telling us that discovering the Der-Mer hookup was ("is") a turning point for Addison, and we see Addy pinning Meredith's missing panties to the lost-and-found bulletin board. Then, a drunken Addison gorges herself on muffins (baked, incidentally, by Izzie) and sobs to an irritated Bailey, who looks like she would rather be anywhere else. Then, a just-showered Addison lets rumpled, apologetic McDreamy into her hotel room. They're both sorry he hooked up with Meredith, and they both agree that their marriage is over. Derek feels this state of affairs is "incredibly sad"…until McSteamy emerges from the bathroom, surrounded by (wait for it) steam, and barely wearing a towel. Then Derek feels much better. Don't we all.