The announcement last month that Barry Allen (Grant Gustin) would speedster his way to National City to hang out with Kara Danvers (Melissa Benoist) sent a wave of excitement through the Supergirl and DC TV fan universe. The prospect initially seemed unlikely considering Supergirl and The Flash air on different networks, but producers Greg Berlanti and Andrew Kreisberg were able to pull off the logistical coup in Supergirl's freshman season and Barry arrives in the nick of time on Monday night.

"You wanted [Supergirl] to work and stand on its own on CBS and have its own audience. We thought we might wait until the second season, but we came back after Christmas and were planning out the final third of the year and we saw a way we thought we could do it," Berlanti explained at an episode screening event. "It felt like the right time for him to enter her world and help her out. It all came together. "

Supergirl: Will The Flash help Kara overcome her Red Kryptonite blues?

Not only did the producers have to get both networks to greenlight the idea, but they logistically had to find a way for Gustin to film the episode in LA while still headlining The Flash, which films in Vancouver. Even juggling all of that, the biggest feat came via the story. Barry shows up organically in Kara's universe at a time that makes perfect sense with her current storyline.

"One of the things that I was most gratified about and got so excited about doing [the crossover], and doing it now, was it wasn't just a gimmick episode. If you're watching Supergirl, [Kara] really needed [Barry] this week because of the things she's been dealing with," Kreisberg explained at the same event.

He then compared Barry's mentoring of Kara to how Arrow's Oliver Queen (Stephen Amell) mentored the Flash in his early hero days. "[I told Grant], '[Oliver] was able to be the voice of reason and help you out. That's what you're going to do for Kara because Kara is just starting out and she's had a crisis of conscience because of the Red Kryptonite episode.'"

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However, fans will have to stretch their imaginations a little after tuning in. The episode of The Flash that properly explains how Barry Allen made it to National City will air on the CW on Tuesday, one night after the crossover event on CBS.

"The audience will have to forgive us [but] they shouldn't wait to watch Supergirl," Berlanti says. "You'll see the shout out in [The Flash] episode with Barry increasing his speed. You have to imagine that there are parallel worlds in different universes and their timeline is slightly different. One is a day ahead."

The Supergirl and The Flash crossover episode airs Monday, March 28 at 8/7c on CBS.

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