As if the big DC Comics four-show, three-part crossover event weren’t a big enough endeavor for all of The CW’s superhero shows, it also happens to be Arrow‘s 100th episode. So to call the crossover a major turning point for the one show that kickstarted the “Arrow-verse” of Arrow, The Flash, Supergirl and Legends of Tomorrow would be a massive understatement.

And that’s why creator/executive producer Greg Berlanti wanted to make sure that the episode was more than merely one third of the crossover story.

“It fits in with the crossover for sure, but it also stands on its own,” Berlanti told Nerdist at the Arrow 100th episode party in Vancouver. “It’s bolstered by a lot of previous episodes of Arrow that have come before. We really wanted to celebrate the show. There is kind of a what-if element to the show in the episode and it’s a very emotional hour for Oliver [Stephen Amell]. I haven’t seen the finished cut of it yet but hopefully it’s as good as we all want it to be.”

But the episode “Invasion!” is more than just a nostalgic look back on the past 99 episodes of Arrow. It also will have a major effect on all the characters moving forward, changing the series as a whole.

“It definitely is a big mark for Oliver’s emotional journey this season,” executive producer Wendy Mericle told Nerdist. “The good thing is, he’s going to go into it in a dark place and he’s going to come out of it with a renewed sense of his mission and a renewed drive to get [season five’s big villain] Prometheus.”

According to Amell, the episode itself is going to be very different from what fans may be expecting to see from such a milestone outing.

“From the storytelling standpoint this year, we really go episode 1, 2, 3, 4, 5, 6, 7, 9. Episode 8 is almost like an event,” Amell told Nerdist. “It’s not really a departure from a storytelling perspective, it’s just that it is really different. I can’t really explain it without spoiling it. It’s such a … well, ‘reach’ is the wrong word. It’s such a departure.”

The four-show, three-part crossover begins tonight at the very end of Supergirl, and then continues Tuesday night as an episode of The Flash and Wednesday night as an episode of Arrow before concluding Thursday night as an episode of Legends of Tomorrow. The catalyst for all these superheroes teaming up is, quite appropriately, something as big as an alien invasion.

“Aliens are where he draws the line,” Amell said with a laugh about Oliver’s reaction to aliens. “He says as much. ‘This is it. That’s it. The line has been drawn.’ Ironically, I say that to an alien. It’s really fun.”

“We use the episode of Legends to deal straight on with Oliver’s reaction to aliens,” executive producer Marc Guggenheim added. “He’s not unfazed by that development, the existence of aliens, and I think his reaction feels very right for his character and feels very Oliver, and is maybe also voicing the thoughts and feelings of fans who started out on this journey with us in a very grounded world and now we’re doing an alien invasion.”

While Oliver’s reaction feels right for him, Diggle’s (David Ramsey) reaction will be perfect for him. “All I can tell you is that Diggle gets to watch a woman fly across the horizon in a skirt,” Ramsey teased to Nerdist. “Television isn’t television unless Diggle gets to react to this, so look forward to that.”

When it came to developing Arrow‘s 100th episode, Guggenheim revealed that the showrunners had a kind of wish list for what they wanted to include in the episode.

“Personally I just wanted to see the show’s past reflected,” Guggenheim said. “I wanted to touch as many moments and lines of dialogue and characters as possible in the episode, within the limitations of characters who are played by actors and some of the actors have gone on to do other things. But I think we came up with a way that keeps pretty much everyone alive which is really nice.”

He continued, “It doesn’t feel so much like a 100th episode as it does the 100th issue of a comic. The storyline is more typical of what you would read in Arrow issue #100 than what you would necessarily watch in Arrow episode 100. Honestly we employed a device that is unusual for a television show but very much in keeping with what a lot of comic books reaching a milestone issue have done.”

According to Mericle, while it was exciting to have the 100th episode as part of the mega-crossover, it also presented new challenges for them.

“We really wanted it to be a look back at the show and something that would honor and be true to the DNA of Arrow. The challenge of that was that it’s also the crossover and the crossover is a way to honor the entire universe,” Mericle said. “So there are some metas here and there and some aliens in this episode, but thanks to Greg Berlanti and his brilliant stroke of genius that he had, he found a way to work with all of that. The episode is going to be great and we’re all really excited about it.”

When speaking with all the actors on Arrow, it was clear that no one ever expected to reach a milestone as big as 100 episodes.

“I’m pinching myself,” Ramsey said with a smile. “I’m a nerd so to be able to do something like this has been the biggest treat in the world. I don’t think I’m speaking too boldly when I say that the success of our show has spawned a whole universe, particularly on The CW and most definitely with Warner Bros. Television. To be a small little piece of that has just been amazing. There’s an enormity to what we’re doing. We’re not curing cancer but we’re entertaining people and changing some lives on some level for the better.”

What are you most excited to see from Arrow‘s 100th episode? Tweet me all your hopes, wishes and dreams at @SydneyBucksbaum!

The 100th episode of Arrow airs Wednesday, Nov. 30 at 8 p.m. on The CW.

Images: The CW