So, who was on the other end of that phone?

That's the big question on everyone's mind at the start of Thursday's Grey's Anatomy - not least of all Meredith (Ellen Pompeo), who immediately jumps to the conclusion that Derek (Patrick Dempsey) is cheating on her after a woman answers his phone. Because the woman's voice is hoofbeats, and a torrid affair is horses, or something like that.

It's the same day as last week's episode in the hospital, and April (Sarah Drew) is so exhilarated by Owen's (Kevin McKidd) over-the-phone surgery tutorial that she grabs Jackson (Jesse Williams) in the hallway and drags him into a supply room to jump his bones. But Jackson kills the mood by pointing out that this would be the first time they've had sex since losing the baby, and April leaves the room in frustration. Jackson needs to work on his pillow talk, sure, but I also wonder if Grey's is setting up a mania storyline for April.

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In a different closet, Owen and Amelia (Caterina Scorsone) are also interrupted in the middle of a hot 'n' heavy makeout session when Owen's pager goes off. And Callie (Sara Ramirez), who is apparently wading into the murky waters of online dating, has cut a first date short because she also received a page. But that doesn't stop the lady she's with from walking her to the hospital and giving her a good night kiss tongue assault. Callie's giddiness doesn't last, however, as she quickly notices that Arizona (Jessica Capshaw) has witnessed the whole thing through a window, and the most awkward elevator ride ever ensues.

Meredith tries calling Derek again, but his phone goes to voicemail. The hoofbeats are getting louder! From a mere "hello," Meredith has deduced that Derek's mystery woman is perky, happy and tall, with good hair, but Alex (Justin Chambers) and Maggie (Kelly McCreary) assure her that Derek isn't cheating. The conversation gets tabled for the time being, because there are more pressing matters to attend to - namely, a husband and wife who are seriously injured after a car smashed into their house.

Callie, Meredith and Stephanie (Jerrika Hinton) - who are trying to keep Meredith's streak alive - team up on the husband, while Arizona, Amelia and Jo (Camilla Luddington) work on the wife, who's pregnant and in much better shape than her husband ... for now.

Elsewhere, Maggie and Ben (Jason George) assess the driver (Dan Lauria, a.k.a. the dad from The Wonder Years!!!!!), who appears to be drunk. He punches Maggie in the face, causing her to fall to the floor. She hits a gurney on the way down and winds up with a nasty cut on her cheek. When the man's wife shows up, she tells the doctors that he has Alzheimer's and must have stolen her keys when she was in the shower.

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As they prepare to operate, it's clear that Maggie has ignored Richard (James Pickens Jr.) and Bailey's (Chandra Wilson) orders to get a CT scan, and it seems like her head injury may be more serious than she's letting on. In the OR, Maggie - who's clearly unaware of Richard's history with Alzheimer's disease (and, therefore, her own) - goes on and on about how Martin's wife should have taken better care of him.

Meanwhile, the pregnant wife is casually chatting with Arizona & Co. about music when she suddenly suffers a seizure out of nowhere and dies. Arizona decides to go ahead and deliver the baby right then and there, and after a few dicey moments, the infant (which Alex names Dan) appears to be fine. He's doing better than Jo, anyway, who bursts into tears and has to leave the room.

While all this is happening, April yells at a patient who's gotten his tongue stuck in a thermos and then tries (unsuccessfully) to jump-start another romp with Jackson, before telling him that she's sick of being treated like a delicate flower.

Meanwhile, in the aftermath of her earlier makeout session, Callie is feeling simultaneously guilty and like she has done nothing wrong, and wondering whether she should apologize to Arizona. "Arizona was seeing other people while the two of you were still married," a bitter, selfish Meredith cruelly points out. Things get more awkward in the NICU, when Arizona presses Callie for details on her truncated date. Instead of apologizing, Callie stands up for herself, telling Arizona that she's moved on and Arizona should, too.

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Realizing that Maggie has no idea her mother suffered from Alzheimer's, Richard asks Meredith if they should tell her about Ellis. But their conversation is interrupted when the wife of Wonder Years Dad (okay, fine, his real name is Martin) tells the doctors that her husband seems to have made a miraculous recovery and has regained his memory.

After running some tests, Amelia discovers that he doesn't have Alzheimer's after all, but instead has a condition called NPH - which does not stand for Neil Patrick Harris, but is a buildup of spinal fluid that puts pressure on the brain. They decide to insert a shunt to prevent his memory loss from happening all over again. But before that happens, the police want to talk to the couple.

Upon learning that he killed a pregnant woman, Martin is distraught. As Amelia tries to hurry him along to the time-sensitive surgery, he drops the bombshell that he doesn't want it. He wants to forget that he's a "murderer." Oh, this is sad. Martin tells his heartbroken wife to put him in a home and let him rot. Ben and Richard argue over whether they should let his wife make the decision for him - but Richard wins out, saying they have a responsibility to honor their patient's wishes.

Meredith, still getting radio silence from Derek, has a heart-to-heart with Callie, Maggie and Alex about whether he's cheating. Calex try to talk Meredith off the ledge - maybe he just lost his phone and some random lady found it! (Unlikely.) Maggie presents a few more plausible (and less depressing) scenarios, like maybe a nurse picked it up while Derek was in surgery. But Meredith points out that this isn't the first time Derek has taken a new job, moved to a new city, and ignored his wife's calls. In fact, that's how they got together in the first place. Eesh, she's got a point ... and Alex and Callie don't have much of a response except to (somewhat inelegantly) insist that MerDer are MFEO, even if Alex won't say "Team MerDer" out loud.

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At least some of the Grey Sloan couples are doing okay, though! Alex talks Jo out of a crisis of confidence about her career choice, and Jackson finally confronts April outside the hospital, saying he's not going to stop worrying about her. They don't even make it out of the parking lot before ripping each other's clothes off. Also, Amelia and Owen reconvene in the on-call room, but their sex is again interrupted by an incoming case. Curses!

Arizona finally confesses the real reason she's so upset about seeing Callie sittin' in a tree with someone else: She used to date "Heather," the woman Callie was with (and whom Arizona now refers to as "Steak Knives"). Callie's clearly heard about this particular ex before, and immediately realizes it's the one who went crazy after Arizona refused to get matching tattoos to celebrate their four-week anniversary (which, to be fair, is like two years in lesbian time). "I wasn't jealous, I was just worried," Arizona tells Callie. Which is kind of a little bit sad, right? But Callie is grateful to her ex-wife for helping her dodge a bullet.

To top off Meredith's terrible, horrible, no good, very bad day, her 89-patient streak looks like it's about to come to an end, since the husband/new father is coding. But Stephanie brings him back from the brink and keeps the streak (and the patient) alive.

Richard decides to tell Maggie that Ellis had Alzheimer's, but Maggie's already had genetic mapping done, and knows that she has a predisposition for Alzheimer's (hence her obsession with doing crosswords). She's mostly relieved to know it doesn't come from Richard - and admits that she might have a concussion after her fall earlier.

And now, the moment we've all been waiting for! Derek shows up at Meredith's doorstep and tells her that he took the first flight after "you called my phone and a woman answered." Way to explain something we already know, Derek. Although I guess this explains why Meredith kept getting his voicemail. Anyway, we still won't get any real answers until next week's episode, because then the credits roll. SIGH.

What do you think, Grey's fans? Is Derek really having an affair? Is April on the verge of a breakdown? And are you upset that Arizona wasn't a little bit jealous of Callie's date? Sound off in the comments!

Grey's Anatomy airs Thursdays at 8/7c on ABC.

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