[Warning: The following contains spoilers for the most recent episode of Arrow.]

The penultimate episode of the massive CW-DC crossover just so happened to coincide with Arrow's 100th episode, which means that in addition to seeing our heroes fight a villainous new alien race, we also got to take a pleasant stroll down memory lane.

After Oliver (Stephen Amell), Thea (Willa Holland), Dig (David Ramsey), Ray (Brandon Routh) and Sara (Caity Lotz) were abducted by the Dominators at the end of Tuesday's episode of The Flash, they were apparently plugged into the matrix. Not the actual matrix from the Keanu Reeves movies, of course, but it might as well have been. Each hero was placed unconscious in an alien pod while they shared a hallucination of what their lives would have been like had Oliver never gotten on the Gambit.

Oliver was happily engaged to Laurel (Katie Cassidy). Moira (Susanna Thompson) and Robert Queen (Jamey Sheridan) were both alive and well. Ray was engaged to Felicity (Emily Bett Rickards). Lance (Paul Blackthrone) was still captain of the Star City police. And Diggle was the vigilante known as the Hood.

Everyone -- maybe with the exception of Diggle -- seemed to be living a more idyllic life, but soon memories of their real lives began to seep through. After some confusion about what in the heck was going on, our heroes banded together to fight their way out of the matrix and back to reality. (Sadly, none of the fight scenes were shot using bullet time.)

But when they woke from their stasis, our heroes weren't out of danger yet. The Dominators had taken them aboard their mother ship, where they had been probing each of them for information while they were unconscious. Fortunately, thanks to some help from Cisco (Carlos Valdes), Barry (Grant Gustin) and Supergirl (Melissa Benoist), the remaining Earth-bound members of Team Arrow were able to track down their location and rescue them with the Waverider, the time-ship the Legends of Tomorrow crew uses to preserve the timeline.

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The bright side of all this is that thanks to their brief time onboard the Dominators' ship, Ray was able to glean some crucial info about their master plan: The Dominators are in the process of creating a mysterious, new weapon, and they're bringing it straight to earth!

Executive producers Marc Guggenheim and Andrew Kreisberg spoke to reporters about the challenges of balancing the four-way crossover with Arrow's 100th episode and Katie Cassidy's emotional return.

What were the challenges in balancing the 100th episode-ness of it all with the crossover story?
Marc
Guggenheim: That was the challenge - just how much to service the 100th episode-ness and how much to service the crossover. I think it's pretty obvious that we chose to go 100th episode, but it was totally a challenge. It was [Greg Berlanti's] idea to have our five Arrow people and basically enter them into -- we call it the matrix, lowercase "M" -- the Dominator matrix, and that allowed us to have our cake and eat it too.

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What was important for you to imbue in the episode and for Oliver to learn out of the experience?
Guggenheim: We make a reference actually in the end of the Legends episode [of] the crossover that this was sort of Oliver's version of Flashpoint and it's a little bit Oliver's version of "The Girl Who Has Everything" from last year's Supergirl. There's a reason why these stories are iconic or familiar tropes, even in comic books. It's because when you show the protagonist the path not taken and you basically put them in the situation where they can choose to stay on that path or go back to their life with all of its ugly aspects and challenges, and they choose the selfless choice of returning to that ugly past, it makes your character stronger because it forced Oliver to double down on his mission and commit to this life with all of its losses and failures and challenges.
Andrew Kreisberg: To answer your question a little more directly, it was getting the cast. Getting Jamey Sheridan, getting Susanna Thompson back, we tried desperately to get Colin Donnell.
Guggenheim: And [Colton Haynes].
Kreisberg: We just couldn't pull it all off. I was just telling Marc, the show's in syndication now and I caught a couple of the early episodes on TNT and we're so immersed in it sometimes it's easy to forget just how far the show has come and how different it used to be. Watching those early episodes and then seeing the 100th and seeing Oliver have scenes with his mom and especially the scene with Thea and Moira, and remembering how important [Moira] was to the mythology and to these characters and to everything that happened. That was the thing that made this feel like a 100th episode. Our first mission was to make sure we locked in those cast [members] and we were fortunate that everybody who has left the show was so excited to come back.

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In regards to that final hologram scene, was that difficult to shoot?
Guggenheim: That beat was not in the original break of the episode. 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 - because 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. And props to our visual effects house. 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 then 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 that makes it sound. They did an incredible, incredible job, as they always do.

It was great to see Katie Cassidy again for the 100th. Can you talk about why you decided to make her such a central figure?
Kreisberg: Because she is such an essential figure to the show. She was the second lead on the show. 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 daughter. Like I was saying, 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." Laurel, 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 then 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.

When we last saw Artemis (Madison McLaughlin), it was revealed that she was someone in league with Prometheus, and she was conspicuously absent from Episode 100. Will we find out where she was while all this was going on?
Guggenheim: It's funny, to be honest with you, in the break of [Episode 8] we talked a lot about, "Do we have her in there?" And it felt like it was the elephant in the room and we didn't want it distracting, so 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 [Episode 7's] cliffhanger with respect to Evelyn big time in [Episode 9].

Legends of Tomorrow will conclude the four-part Arrowverse crossover Thursday at 8/7c on the CW. Arrow airs Wednesday nights at 8/7c on the CW.

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