If you haven’t muttered “I’m done with this show” while reluctantly pouring yourself a second glass of something, anything, on a Thursday night — fully knowing, of course, that you’ll do the same thing next week — you can’t really call yourself a Grey’s Anatomy fan. Sorry, but those are just the rules.

Tuning in week after week for 15 years with the knowledge that utter emotional devastation is not only probable but inevitable — a plane crash will kill off most of the main cast! Someone will sneak out in the middle of the night to go live in Kansas with their ex! — takes its toll.

The list of unforgivable plot lines on the medical drama is as long as a Miranda Bailey monologue to new interns, but one of the most divisive — and shall we say, fascinating — Grey’s plots has to be the Izzie Stevens and Denny Duquette “ghost sex” episode in Season 5.

The episode had everything. Banging an apparition notwithstanding, it also featured Mark Sloan figuring out that he has a crush on Little Grey, which was the beginning of one of the series’ most gut-wrenching romances; the interns practiced skills on themselves; Meredith Grey and Cristina Yang had an epic fight about their careers; and the banter between Meredith and Alex Karev (“What, you think you're the only one who paid a nurse to have her page you with an appy?”) is just… it’s the kind of dialogue that makes you forget you’re about to watch Izzie sleep with a ghost.

Which, of course, doesn't make a whole lot of sense. But that's kind of what we love most about Grey's, isn't it?

The episode, officially titled “In The Midnight Hour,” was directed by Tom Verica. (If you recognize Verica’s name, it might be because he played Sam in How To Get Away With Murder). Verica had to dig deep in the archives of his memory to recall the iconic 43 minutes in Grey’s history — the episode aired November 20, 2008 — but he remembers it, and that era of Grey’s, fondly.

“We just had a great time,” Verica tells me over the phone about filming “In The Midnight Hour.” “I can see, watching back the episode, the moments where we would yell ‘cut’ and just break up laughing because we were trying to wrap [our heads] around, psychologically, is [Izzie] having a [psychotic] break here, is she losing her mind...”

Which are great questions. It's almost reassuring to hear that those on-set were asking themselves the same things we viewers at home were. And it's precisely this confusion that made the episode so polarizing.

We didn’t know if Izzie’s visions of Denny, and their seemingly very-real sexual encounter, was evidence 1) of a temporary, grief-driven mental breakdown, 2) of something even more serious, or 3) that the show had now... cast a ghost as a main character. (We later discovered it was something like a combination of all three, but with an emphasis on number two.)

But! We missed some hints that would've helped us figure out what this ghost-sex scene was really all about. Verica says he may have had an inclination about what was going on with Izzie and her yet-to-be-discovered brain tumor, “but of course,” he says, “I just had to address what we had in the here and now.” So he tried to help the viewers suss out the truth behind the ghost-sex using small, burning flames.

Verica tells me that the candles in Izzie’s bedroom offered a clue as to her mental state. “We were specific about where you saw those candles and where you didn't,” he says.

When you see the candles, it speaks to her state of mind when she was “caught up in it, not questioning herself and the reality of the situation, as she was in the earlier scenes or the later scenes.”

When we don’t see the candles in those earlier and later scenes Verica’s talking about, Izzie understands that seeing Denny, and having sex with him, can’t be real. He’s dead! She couldn’t save him! She has a life now without him, and a job, and a boyfriend! What is he doing here?! And how dare he apologize for being dead.

The visual clues were lost on me (and probably most of us?) at the time, but the reality vs. fantasy theme in the episode was palpable, and it had to be approached carefully. They wanted to play with the idea of Izzie’s struggle, Verica says, and that’s why they needed to light those candles and tousle her hair a little, so that viewers would actually be wondering, alongside Izzie herself, if she’s even interacting with the ghost at all.

You see, though, it’s that wondering that frustrated a lot of fans; the ghost-sex was a real dealbreaker for viewers at the time and remains so to this day. Jessica Shen, a moderator of the popular Grey’s Anatomy Facebook group for fans tells me that when she does her rewatches of the series, she’ll often skip Season 5 just because of the whole Izzie and Denny ghost story arc. “The ‘ghost sex’ was rock bottom,” she tells me.

Me, you, Shen, and millions of other fans were kept in limbo wondering what was really going on with Izzie and Denny for a long time — from Episode 8 until the tumor reveal much later in Episode 17. It was a lot of weeks of trying to decide whether you still wanted to watch this show or cared if Izzie was cured of whatever was causing these visions… or killed off.

Because, remember, behind the scenes, there was real-life gossip that Katherine Heigl wanted out. Heigl was publicly fighting with the show’s producers and even removed her name from the nomination ballot for the 2008 Emmys (this happened the summer before the ghost-sex arc aired) because she didn’t think the writers were doing her justice in Season 4.

Entertainment Weekly reported at the time that the bonkers ghost-Denny/cancer plot line in Season 5 was an attempt to let Heigl show off her acting chops, but critics and fans were on Heigl “deathwatch,” waiting for her to be killed off — possibly by, what else, a brain tumor — because of all the reported tension backstage. (She left the series in Season 6.)

Behind the scenes drama aside, Heigl and actor Jeffrey Dean Morgan (Denny), “jumped in” and really went for it “In The Midnight Hour,” according to Verica.

“I directed on the show frequently, but I never had too much stuff with him until that episode,” Verica says of working with Morgan. “He was terrific. It was awkward for him because he's playing this image and we really had to work to ground it … to give it that connection, that real connection that they had.”

Ghost or no ghost, Heigl and Morgan had chemistry. “They were very good together,” Verica says. “They had a very easy working relationship.”

Because Verica had starred with Heigl just two episodes before — he played hospital patient Mike Norris in Episode 7, the man whose heart Izzie tried to steal in Season 2 for Denny — he says he had a great relationship with Heigl, too. It was “a blast,” he says, and filming the episode was smooth, too, given that they filmed all of Heigl and Morgan’s scenes at the house in one day. “All her work, we got to play the whole arc, with exception back in the hospital, of that storyline right there.”

“In The Midnight Hour,” despite the fan frustration it caused, might be an archetypal Grey’s episode; it’s divisive but delightful, hilariously unrealistic but somehow grounded in reality, absolutely ridiculous but perfectly approached.

“I think that's quintessentially the success of the series, is being able to navigate the humor and the emotion and the heartbreak,” Verica says, adding that “keeping the levity in it wherever possible” was also clutch.

“Look, I remember the first episode I directed was all about penis fish, which was like, ‘Is this a real thing?" (The penis fish episode aired toward the end of Season 3.) Turns out it actually is — having a fish swim up your urine stream and into your prostatic urethra is not the most common thing, but much like the symptoms of Izzie’s brain tumor that cause her to hallucinate a phantom tryst, it’s something that could, theoretically, happen in real life. Is it likely? Nope, not at all. Possible? Yes.

Verica says that they really straddled that line with how they pushed the characters, and put them in situations where they have to deal with nearly unfathomable medical and interpersonal scenarios.

But that’s the beauty about Grey’s, you have to just accept surprise after surprise, whether you like it or not. From sex with a ghost to penis fish to patients with tree hands, these things have close to a zero percent chance of happening, but, Grey’s convinces you that they might. And convinces you to yep, tune in next Thursday, cocktail or bottle in hand.

And when you add a little chemistry in that cocktail, Verica says, “you've got a great episode of Grey's Anatomy.”

Images: ABC; screenshots via Netflix