Executive producers Greg Berlanti and Andrew Kreisberg break down why this crossover is happening on CBS' 'Supergirl' instead of The CW's 'The Flash.'

The planets aligned almost perfectly to make the Supergirl crossover episode with The Flash possible.

While the cross-network DC TV crossover on CBS came to fruition despite many obstacles, executive producers Greg Berlanti and Andrew Kreisberg reveal that there is still one hiccup in how the story will affect The Flash moving forward.

The episodes are airing out of order.

While the crossover is airing as an episode of Supergirl on Monday, "Worlds Finest," the events that Barry Allen (Grant Gustin) lives through in National City with Kara Danvers (Melissa Benoist) will carry over to Tuesday's episode of The Flash, "Flash Back." But The Flash episode technically comes first.

"They flip order, obviously, because this will air Monday and The Flash one that sort of hints at how he gets over there and gets back airs on Tuesday," Berlanti told reporters Thursday after an early screening. "The audience has to forgive us because one has to air before the other, but they shouldn't wait to watch Supergirl until after The Flash episode. They should certainly watch it before. But you'll see the shout out in the episode. It deals with Barry increasing his speed and we're very excited about next week's episode of The Flash, too."

According to Berlanti, the episodes airing out of order is just the price they had to pay when dealing with two shows on two different networks.

"Had it been all on one network, we would have been able to coordinate that," Berlanti says. "I'm actually surprised we were able to do it as closely as we were. That worked out to that degree. So you have to imagine they're in parallel worlds, in different universes and the timeline is slightly different. One day ahead."

"When you binge-watch it, it will all make sense," Kreisberg adds with a laugh.

Here are a few other things to know about the upcoming crossover, courtesy of Berlanti and Kreisberg.

Why is the crossover an episode of Supergirl instead of The Flash?

Berlanti first tried to answer by explaining that it's because Barry's abilities allow him to travel to Kara's universe, before admitting they didn't actually consider the other option.

"It's an interesting question, actually," Berlanti says with a laugh. "We never really considered the other way. I don't think we even talked about that. I think it's a little more fun at first to bring the veteran from that show to the chemistry of a new show."

Could Kara ever show up in Central City on The Flash?

The showrunners aren't ruling out another crossover the other way around.

"We would love that," Berlanti says. "We're waiting to see the audience reaction from something like this."

Why is Barry the only CW superhero crossing over? Why not anyone from Arrow?

It all boils down to the fact that Barry's abilities allow him to travel throughout the multiverse.

"[Barry] was the door, the gateway," Berlanti says. "He's the only one who had the capacity to open that door, and hopefully now it's [staying] open. In a perfect world, it probably would have been both of them [Gustin and Arrow's Stephen Amell]. But logistically that would have been a nightmare to try and do both shows. We had to facilitate one."

The showrunners were also intrigued by how "similar" Gustin and Benoist are as actors.

"Barry and Kara both have a light to them," Kreisberg says. "They're definitely heroes that are daytime heroes that live more in the light. Watching the two of them form this instantaneous best friendship was really exciting for us to explore in this episode."

What is Barry doing in National City?

While trying to increase his speed while wearing a tachyon generator strapped to his chest, Barry accidentally runs too fast and ends up traveling to a different universe. Not Earth-3 specifically, but another Earth where the Flash and Arrow and all their teammates don't exist. While stuck in National City, Barry ends up making himself useful to Kara and the DEO.

"It's not just a gimmick episode," Kreisberg says. "She really needs him this week because of the things that she's been dealing with. Kara is just starting out and she's had a crisis of conscience because of the red kryptonite episode. I always think back to 'Hush,' from Buffy. On its own, that's the one where nobody could hear anything and nobody could speak. But it was more than that. The Flash coming to Supergirl really helps Supergirl overcome the challenge that she had been dealing with. His advice to her about how she should be living her life is about to have repercussions as far as Jimmy [Mehcad Brooks] is concerned."

How did this crossover even happen?

The showrunners truly did listen to what the people wanted.

"One of the things that really kept the flame and the torch for it alive was a lot of journalists and the fans online, that consistent feedback throughout the year and enthusiasm for the idea," Berlanti says. "We always wanted to do it, from the day we cast Melissa. It was exciting to see them onscreen together. But a few things had to go right. We were introducing the multiverse on The Flash and that had to work."

The DC TV bosses have always been clear about the fact that Arrow, The Flash and Legends of Tomorrow exist in a different universe than Supergirl, so they had to establish a clear reason why Barry and Kara would meet.

"Flash and Arrow exist in a universe where there is no Superman or you would have heard about him, and certainly Supergirl as well," Berlanti says. "And then there was the notion that it was two separate networks so it was definitely more challenging in that way because it's not convenient. It's two separate cities that they shoot in. And you wanted the story to work and stand on its own on CBS, have its own audience."

Why is this crossover happening now?

All the showrunners initially thought they would have to wait to do a crossover until next season.

"After Christmas we came back and were planning out the back half of the year, and we saw a way that we thought we could do it," Berlanti says. "It felt like the right time for him to enter her world and help her out. It all came together. Then there was navigating the logistics of having Grant coming down not just once but twice, right when he's in the heat of the season up there and having to make that happen. And then the construct of the episode itself."

Supergirl airs Mondays at 8 p.m. on CBS and The Flash airs Tuesdays at 8 p.m. on The CW.