6 Things to Know About The CW’s Epic ‘Arrow’verse Crossover

The enormous Supergirl-The Flash-Arrow-DC’s Legends of Tomorrow crossover has officially kicked off on The CW.

At the end of Monday’s Supergirl, The Flash‘s Barry (Grant Gustin) and Cisco (Carlos Valdes) arrived on Kara’s (Melissa Benoist) Earth to ask for her help defeating a new nemesis. Naturally, she agreed…but the scope of their mission was still very much up in the air.

Arrowverse executive producers Andrew Kreisberg and Marc Guggenheim, and stars Stephen Amell, Caity Lotz and Victor Garber shared what fans can expect during the heroes’ biggest test yet.

It’s Heroes v. Aliens!

Last year’s crossover event served as a launching pad for the first season of Legends, and featured that show’s Big Bad Vandal Savage (Casper Crump). This go-round, “we all collectively wanted the superheroes to face an external threat…a threat that came from outside of the shows,” Guggenheim said of the decision to bring in the Dominators. “[Executive producer] Greg [Berlanti] just walked in one day and was like, ‘Let’s do Invasion.’ I think [Kreisberg and I] both had the exact same twin reactions: ‘That’s totally awesome’ and ‘How the hell are we going to do that?'”

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The scheduling of the episodes required Supergirl‘s hour to be (mostly) standalone from the crossover event.

Though the Flash, Arrow, and Legends installments are all-in on the crossover, the Supergirl episode, “Medusa,” was primarily a normal hour of the series until Barry and Cisco made contact with Kara. That decision came when the writers realized the Supergirl portion would be the show’s final episode of 2016. “The other shows—Flash, Arrow and Legends—all have a subsequent episodes [after] the crossovers,” Kreisberg told reporters after a screening of the superhero team-up. “With Supergirl, it was our midseason finale. So we wanted to make sure that Episode 8 of Supergirl spoke to what had been happening on the first seven episodes of [Season 2] and not just as a tie-in.”

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As it was, this crossover was already considerably bigger than what the shows have pulled off in previous years. “Last year we did two shows with a sprinkling of Legends,” Kreisberg pointed out. “This year, we went to three shows. The idea of going to four full shows, I’m not sure we could have pulled off logistically speaking…hopefully people didn’t realize it, but Kara was not in a lot of Episode 7 of  Supergirl, where she was captured by Cadmus, specifically to free her up for five days so that she could appear in all these other episodes.”

The addition of Kara to the team causes a stir.

Supergirl is the new kid on the block to most of her fellow superheroes, which leads to tension, flirtation and playful banter from her new teammates. And though Kara’s skills are useful when it comes to training to fight the Dominators, some simply don’t know what to make of their alien ally. “I think [Sara] is intimidated by her, at first, because she’s used to being like the bad bitch in town,” Lotz explained of her character, aka White Canary. “And then all of sudden, [here] comes this alien girl who can like levitate and burn things with her eyes. It’s kind of like, ‘OK, well…’ I think it’s getting over that intimidation and needing to go, ‘OK, I see you.'” But ultimately, “she’s impressed by her.”

Not everyone with powers will be allowed to fight.

To defeat the Dominators, everyone will have to step up…but Team Flash is reluctant to include Wally (Keiynan Lonsdale), who has only recently gained speed abilities. The Flash‘s take on “Invasion” puts a strong focus on the crossover, but they were also able to organically continue Wally’s growth in the hour. “One of the things we always try to do, especially with the crossovers, is not make it feel like all of a sudden we’re in this evergreen world that’s completely disconnected to everything that’s been happening previous,” Kreisberg said. “At the end of [The Flash] episode 307, Wally became a speedster. That’s huge; that’s big news. To sort of gloss over that and not have anybody react, considering the myriad of reactions Joe has had to Barry becoming a speedster and Iris finding out—it felt like we’d be doing a disservice to the people who were watching The Flash and enjoying it.”

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Will Future Barry’s message be addressed?

Legends‘ Firestorm duo of Martin Stein (Garber) and Jax (Franz Drameh) are keeping their own secret from their team: Earlier this season, they found a warning message from a future version of Barry. But now that they’re teaming up with him, will it remain a secret? “The fact that this is coming from Barry years later…he finds out,” Garber teased. “So it sort of unfolds, as things do on these shows. And lots of things occurred in that time period.”

The crossover allows for a unique Arrow milestone.

As the four CW/DC shows come together to put on their biggest crossover yet, Arrow is also celebrating an impressive milestone: its 100th episode. Thanks to the circumstance of the crossover, the hour explores a different path Oliver (Amell) could have gone down…with a twist. “To explain the 100th episode is impossible because it is such a radical departure from anything that we have ever done on the show,” Amell said. “But we utilize the fantastical elements that we bring in on The Flash and carry through to Legends of Tomorrow with the present day storyline to create something for the 100th episode of the show that…basically gives us license to do whatever we want and to show some things that we’ll never be able to show on the show, to have some characters interact that will never be able to interact on the show again.”

RELATED: Arrow‘s 100th Episode Brings Back Old Friends

The big moment coming during the show’s annual crossover also allowed for the full impact of what Arrow has achieved to sink in. “We started as one show, we are now four shows in a universe that has developed on a network,” Amell said. “I never have the ability to see the macro of what we’ve done, because I’m always in the weeds so much, but there was really a moment on the 100th episode where I took a deep breath and was like ‘This is really cool.’ We went around and looked at all the different people that have been there since the beginning and shared stories and it was very, very cool.”

The Flash, Tuesday, November 29, 8/7c, The CW

Arrow, Wednesday, November 30, 8/7c, The CW

DC’s Legends of Tomorrow, Thursday, December 1, 8/7c, The CW