Warning: the following story contains major spoilers from Arrow’s 100th episode, “Invasion!” Don’t keep reading unless you’ve caught up to the Arrow part of the big crossover with The Flash, Supergirl, and Legends of Tomorrow. You’ve been warned …

Sometimes the best way to move forward is to look back, and Arrow proved that with its milestone 100th episode.

In the second hour of the big DC TV crossover with Supergirl, The Flash, and Legends of Tomorrow, five characters—all originally from Arrow before some moved on to the other DC shows—found themselves in an alternative reality where Oliver Queen (Stephen Amell) never got on the Queen’s Gambit, therefore never becoming the Green Arrow. The invading alien race the Dominators captured the five humans from the group fighting them—Oliver, Thea (Willa Holland), Diggle (David Ramsey), Ray Palmer (Brandon Routh), and Sara Lance (Caity Lotz)—to get information from their minds about the metahumans on their team. After each character figured out their peaceful, happy lives were not actually real, and just a shared hallucination, they decided to fight against it and get back to the real world. Thea, however, chose to stay behind at first, not wanting to lose her parents Robert (Jamey Sheridan) and Moira (Susanna Thompson) all over again. But in the end she chose her brother Oliver over the memory of her parents and left the fake reality with him.

Before Oliver escaped the alternative reality, though, he got a glimpse of all his loved ones, past and present, one last time. Standing in hologram form before him were his parents, Laurel Lance (Katie Cassidy), Tommy Merlyn (Colin Donnell), Roy Harper (Colton Haynes), and Felicity (Emily Bett Rickards). Seeing them all smiling at him recalled memories of things they said to him over the years, and was an extremely emotional and touching moment for the man who used to keep everyone at arms’ length.

“That beat was not in the original break of the episode,” executive producer Marc Guggenheim told Nerdist along with a small group of reporters after screening the episode early. “Honestly, I can’t even tell you where that idea came from. It was just I was writing that scene where Oliver goes back and it just popped into my head that basically all the actors that we can’t get, at the time I didn’t know who we could get and who we couldn’t, I’m like this is a way we can get them.”

While Arrow was able to get Cassidy, Thompson, and Sheridan back for the nostalgic 100th episode, the show was unable to get Donnell and Haynes due to scheduling purposes (although the episode did make a hilarious shout out to Donnell’s new series regular gig on Chicago Med!). Their holograms were completely CG as a way to still include them in the milestone episode.

“Props to our visual effects house,” Guggenheim said. “Zoic handled those shots and they did an amazing, amazing job, particularly with Colin and with Colton, because we couldn’t reshoot them. We had to take them from old episodes and roto them out and put them into this. It was hard, obviously, because they had to work with pre-existing footage. Yes, they had 99 episodes to choose from, but it was a lot harder than it makes it sound. They did an incredible, incredible job, as they always do.”

While most of the entire episode took place in the Dominators fake world created in the minds of Oliver, Thea, Dig, Sara, and Ray, expect there to be major fallout and ramifications from what they all went through. And not just for the last episode of the crossover on Legends of Tomorrow but also for Arrow as a series moving forward.

“With respect to Arrow, going into 509, which is the Arrow midseason finale, you’ll see that Oliver sort of has a new sense of purpose,” Guggenheim said. “The events of 508 basically forced him to emotionally double down on his mission, so he goes into 509 with a new sense of purpose. I would say, also, a reaffirmation of his bond with Thea, because they basically chose each other in 508. That carries through the midseason finale and, obviously, sets up things beyond it.”

Another way the crossover is going to affect Arrow moving forward is the fact that everyone is now clued in to the fact that Barry Allen (Grant Gustin) changed the timeline on The Flash, and how that had repercussions for everyone in the present.

“Arrow 509, the midseason finale, there’s a fair amount of discussion about Flashpoint, given the fact that, essentially, the crossover outed Flashpoint to the Arrow characters who weren’t Felicity,” Guggenheim said. “They deal with—I think in some humorous ways, actually—some of the ramifications. For example, I think Curtis [Echo Kellum] is concerned that maybe he was straight originally, as one example.”

One reason why the showrunners were so excited about doing a nostalgic hour for Arrow‘s 100th episode was because they got to bring back so many integral characters that are no longer on the series, like Laurel.

“She is such an essential figure to the show. She was the second lead on the show,” executive producer Andrew Kreisberg said. “Even though the show has evolved, Laurel is at the heart of it. She was Oliver’s great love, she’s Sara’s sister, she’s Lance’s [Paul Blackthorne] daughter. Watching those early episodes, so much of it revolved around her relationship with all these characters. I watched the episode ‘Vertigo’ where she’s arguing with her father about how much Thea reminds her of Sara and how he should go easy on her. I’m sitting there listening to that and thinking about, ‘God, they didn’t even know Sara was alive yet.'”

He continued, “Even if we do get to 200 episodes, Laurel will always be at the heart of the series and be such an important character and, on top of that, Katie Cassidy will always be so important to us. We were so happy that she agreed to come back because she is part of the family, both behind the camera and in front of it.”

With so many fan-favorite characters returning for the episode, one current character was conspicously absent: Evelyn Shaw aka Artemis (Madison McLaughlin). In the previous Arrow episode, it was revealed that she was in contact with the big bad villain Prometheus, though it’s not yet known the extent of her relationship with him or her betrayal of Team Arrow.

“It’s funny, to be honest with you, in the break of 508, we talked a lot about of do we have her in there, and it felt like it was the elephant in the room and we didn’t want it distracting,” Guggenheim said of Artemis being left out of the episode. “We don’t reference it in large part because I’m not a fan of, ‘Oh, it’s too bad that Evelyn’s mom is sick this week.’ I would prefer to just whistle past the graveyard. I will say, you will get a payoff to 507’s cliffhanger with respect to Evelyn big time in 509.”

Since there was so much to pack in to just one episode, some scenes ended up being cut for time.

“There was a little exchange between Sara and Kara [Melissa Benoist] that I really liked—I don’t think we even filmed it—where Sara says, ‘Hey do you want to get a drink when this is all over,’ and Kara says, ‘I think you wanna meet my sister,'” Kreisberg said. “Just the idea of starting the Sara/Alex [Chyler Leigh] ‘shippers going. It’s those little moments that wound up on the cutting room floor.”

Another one that got cut was a “funny scene” between Speedy and HR (Tom Cavanagh).

“She says, ‘So what are you doing here? What value do you bring?’ He’s like, ‘Well I’m writing a book,’ and she’s like, ‘Oh that’ll be a big help,’ and walks away,” Kreisberg said with a laugh. “It was all of those little tiny moments. It’s amazing how many of them we actually got to keep, because this episode came in wildly over, not surprising. And you’ve got to keep the plot going, and you had to have room, especially in these episodes, which probably had even grander visual effects sequences than we’re used to in an average episode, so it tended to be those little jokey moments that fell by the wayside.”

“There was a Supergirl moment on the Waverider that I really wanted to get in there,” Guggenheim added. “I’m not going to tell you what it is, because we may do it next year, but Supergirl never ended up on the Waverider so we couldn’t do the moment. And obviously there were moments with Colton and Colin in Arrow that were scripted that we obviously just couldn’t do because of Colin and Colton’s availabilities. For example, Roy was going to be Thea’s boyfriend—that hadn’t changed—and they met when he stole her purse, and that also hadn’t changed. I just thought that would have been fun and nice to see.”

And Kreisberg added, “So, the 200th!”

What did you think of Arrow‘s 100th episode? Tweet me your thoughts at @SydneyBucksbaum!

Arrow airs Wednesdays at 8 p.m. on The CW. The DC TV crossover concludes tomorrow with Legends of Tomorrow at 8 p.m.

Images: The CW