Supergirl type TV Show network The CW genre Superhero Where to watch Close Streaming Options

“This is crazy. What are we doing? What are we wearing? Where are we? This is so cool.”

That was the reaction Melissa Benoist recalls upon filming the first take of the long-awaited crossover episode between The CW’s runaway hit The Flash and CBS’ promising newcomer Supergirl. As the Girl of Steel, Benoist has had 17 episodes of flying on wires and battling alien baddies under her cape. But it’s still quite a sight standing opposite former Glee castmate Grant Gustin in Flash’s maroon leather suit on location in downtown Los Angeles, where the superfriends prepare to battle a duo of silver-­haired villains hell-bent on Supergirl’s destruction. So just how did the Scarlet Speedster and the Maiden of Might cross paths? In short, superspeed. But let’s go back to the beginning …

From the second Supergirl was announced for a fall 2015 premiere, fans speculated about whether Kara Danvers existed in the same fictional universe as executive producer Greg Ber­lanti’s other TV superheroes. While sharing characters across a night is relatively easy (see: NBC’s Chicago Fire/P.D./Med), taking them across networks is slightly more difficult. Helping the case for a crossover, though, both Supergirl (which airs Mondays at 8 p.m.) and The Flash (Tuesdays at 8 p.m.) hail from Warner Bros., which, with CBS, owns The CW. “Everybody was on board, because everybody knows that these things are special,” says Andrew Kreisberg, an executive producer on both shows. The real challenge was the wait. It was important to all involved that Supergirl establish herself as her own hero, and equally crucial to introduce the idea of parallel universes, which The Flash has spent a bulk of its sophomore season doing. “If Supergirl and Flash were existing in the same universe, then why has no one on Flash or Arrow ever talked about Metropolis or the fact that there’s a Superman?” Kreisberg points out. “The one explanation for that is because he doesn’t exist in that world.”

So how exactly does the crossover work? For the uninitiated, Barry/The Flash, who has been training to increase his already supersonic speed in order to fight übervillain Zoom, uses a tachyon device that causes him to run so fast he literally ends up in an alternate universe. Therein lies the reason why it’s a lot easier to bring Barry Allen to a different Earth than the less plausible un-super superhero Oliver Queen (Stephen Amell). While the trip will amount to a hot second on the April 5 episode of Flash, it results in Barry spending a day in National City (and on CBS for the March 28 episode of Supergirl), where he’s shocked to discover another hero. “Supergirl has never heard of The Flash and Flash has never heard of Supergirl, so there’s a lot of like, ‘Wait, you’re the who, now?’ ” says Kreisberg. The comedy, in that sense, hits the ground running with the classic tale of boy saves girl, girl miraculously flies away, didn’t need saving in the first place. “Barry and Kara take a little bit of a leave of their problems for a week to engage in the kind of hilarious high-stakes high jinks that ensue when two super­heroes wind up in the same universe,” Kreisberg says.

Image zoom Michael Yarish/CBS

Therefore, unlike the previous and typically darker Berlanti-verse crossovers — like when Barry was whammied into an evil rage speedster or that time literally everyone died at the hands of Legends of Tomorrow baddie Vandal Savage — these heroes are quick to trust each other. Both lost parents at a young age, but they’re fun-loving, geeky and they haven’t let their pasts dim the light inside them. “What’s so wonderful about these two characters in particular is that they’re so joyful and happy to be heroes,” Benoist says. “There’s this mutual understanding, respect, and excitement that they found each other.”

It’s a relationship that mirrors the kinship between Benoist and Gustin, who swap high fives between takes like old pals. “I hear nothing but amazing things about Melissa all the time — what a hard worker she is and how positive she is,” says Gustin. “It’s all true.” Supergirl’s Southern California shoot location doesn’t hurt either. “Almost everything has been in the sun, which is very different than when we do Flarrow,” Gustin says, referring to the Vancouver-based Flash and Arrow production. “It’s just lighter, brighter, and a different type of energy.”

Narratively, the timing of Barry’s arrival in National City is a fortuitous one: After burning all her relationships during a bout with inhibition-freeing red kryptonite, Kara has hit a low point in her career as a hero. “She’s fallen out of good graces with the people at National City,” Kreisberg says. “The Fastest Man Alive’s advice, ironically, is to slow down and let things come to you.” In that sense, Barry steps into the Emerald Archer’s super shoes as the seasoned vet. “He teaches her quite a bit about what it means to be a hero, what kind of hero she wants to be, and even personally about love, friendship, and being true to who you are,” Benoist says. “It feels really good for her to have someone to relate to.”

In turn, Barry’s experience in National City leaves him with a new sense of purpose upon returning to fight Zoom, recently revealed to be Hunter Zolomon (Teddy Sears). “He’s able to realize that he has more experience than he gives himself credit for,” Gustin says. “That makes him come back with new confidence and this happiness because of this new friend that’s out there.” And possibly a love interest? “We tease it a little bit,” Gustin says. “He’s in town for a day so there’s not really time for anything to actually develop, but there’s undeniable chemistry, and they have a lot of things in common that they’ll never have in common with anybody else, obviously. This is the first person with powers Barry [has met] who’s using them for good. It happens to be somebody that he is attracted to, so it’s cool.”

For what it’s worth, Barry is quick to sense the dynamic between Kara and James (Mehcad Brooks), and even imparts some sage love advice. “Part of the fun is watching Winn [Jeremy Jordan] and James react to suddenly seeing Kara’s got a new friend who’s a superhero,” Kreisberg says. “Winn thinks all things Multiverse are cool and wants to be Barry’s best friend. Meanwhile, James Olsen is a little bit like, ‘Hmm, I didn’t really have any competition until the guy with superpowers showed up.'”

Even more comedy comes from Barry’s reaction to Kara’s boss. “He’s kind of intimidated by Cat [Calista Flockhart], because he doesn’t know how to feel her out at all,” Gustin says. “There’s a scene where Barry goes to CatCo, he’s with the whole crew, he meets Cat and he’s thrown off by her and doesn’t really know how to approach her. Kara is so intimidated by her and she’s a superhero! It’s like, ‘I don’t get this lady, she’s kind of scary.’ There’s no dynamic like that on Flash right now, so that was fun.”

Image zoom Robert Voets/CBS

As much as fans have been championing the marriage of might, so too are studio execs hoping for a fruitful relationship between the two properties. Flash’s sophomore season is averaging 5.7 million viewers, which makes it the network’s No. 1 series, while Supergirl’s numbers have been mostly on the decline since its October debut, putting it in the bottom half of CBS’ programming in total viewers and 8th in the coveted 18–49 demo. “We’re trying to get people in to see this wonderful woman,” says Supergirl EP Ali Adler, who previously worked with both ­Benoist and Gustin on Glee. “Obviously The Flash is very successful and brings a whole world from The CW over to ours. We’d love to have them attend our party, too.”

But it wouldn’t be a real party without an epic showdown. Bringing together two heroes facilitates the need for double the trouble, which is why returning villain Live Wire (Brit Morgan) and emerging baddie Silver Banshee, a.k.a. Siobhan Smythe (Italia Ricci), are holding Cat Grant captive in the heart of National City, the location of the final showdown between the quartet of heroes and villains. “When Siobhan discovers that there are two heroes in town now, The Flash and Supergirl, she realizes she’s going to need a cohort if she’s going to take them out,” Kreisberg says. “It does remind me of The New Batman Adventures animated show, like ‘Girls’ Night Out,’ where Livewire teams up with Ivy and Harley, and you had that fun camaraderie between girls who were loving watching who can be the baddest of the bunch.” In short, both. This is actually Barry and Kara’s second attempt at the taking down the villains and neither outing has gone particularly well. “Throwing lightning at somebody that can manipulate electricity isn’t the brightest of ideas,” says Gustin, who teases the super-duo will use a piece of Flash tech to even the score with Silver Banshee’s also impressive screeching skills.

Image zoom Robert Voets/CBS

The supers have one thing going for them, though: The dastardly duo aren’t criminal masterminds, especially with Silver Banshee just coming into her own. “Banshee’s under Livewire’s wing a little bit,” says Morgan, who teases that the former CatCo employee returns even more vengeful after stewing in prison for months, causing her to go a little crazy. “Livewire’s teaching Siobhan how to be a villain. She’s helping her make the costume and Banshee’s cool with that. As long as the power is still in Livewire’s hands, they’re good. I think sound and electricity goes well together.” With that said, “We know who’s probably going to win this one,” Ricci adds with a laugh. “We’re not watching Banshee on CBS at 8.”

But viewers will get to watch Barry and Kara address an age-old question. Like The Flash and Superman before them in the comics, the two have a footrace to figure out who is faster. “It’s one of my favorite moments in this episode,” says Kreisberg. “It’s close.” Hopefully close enough that they’ll need a rematch next season, maybe with Green Arrow and the Legends as an audience. “That’s in the cards at this point,” says Gustin. “The whole point of us doing this crossover is setting up the fact that maybe next year we can cross over all of them.”

The Flash-Supergirl crossover airs Monday, March 28 at 8 p.m. ET on CBS.

This story originally appeared in issue #1407 of Entertainment Weekly, on newsstands now or available digitally here.