For fans that have been patiently waiting to see Melissa Benoist’s Supergirl team up with one of the other heroes of the DCTV shared universe, the time hasn’t exactly passed in a flash. It’s fitting, then, that when they finally get their wish in “Worlds Finest,” tonight’s special Supergirl crossover episode, the hero that Kara will find herself fighting alongside is…well, the Flash.

It makes a lot of sense. Of Kara’s fellow DCTV heroes, the Flash shares her largely optimistic outlook and cheerful demeanor. There’s also the obvious speed factor. Both heroes possess super-speed, and have a long comic book history of racing each other to see who’s the fastest—a tradition that tonight’s episode pays homage to. But it’s Barry’s newfound ability to access alternate Earths within the Multiverse that sealed the deal for the show’s producers.

“I think for me, he was the gateway,” explains Greg Berlanti, Executive Producer of both Supergirl and The Flash. “He was the only one who had the capacity to really open that door between worlds, and hopefully now it’s open for good. In a perfect world, it probably would have been both the Flash and Green Arrow. But that would have been a nightmare logistically at this point in the year, so we had to go with one.”

“Creatively, with the Flash and the Arrow crossovers, they’ve always been designed to highlight the heroes’ differences, so those episodes have tended to have a great deal of conflict,” adds fellow Flash and Supergirl Executive Producer Andrew Kreisberg. “What was appealing about this is that Melissa and Grant are so similar as actors and Barry and Kara both have a light to them. They’re definitely heroes that live more in the light, and watching the two of them form this instantaneous best friendship was, creatively speaking, really exciting for us to explore in this episode.”

From the moment it was revealed that there was a Supergirl series in production, fans have been asking about a crossover. While the desire and appeal is understandable, the reality of making crossover episodes happen is always a challenge. For Supergirl and The Flash, the challenges were even greater than for The Flash and Arrow since Supergirl airs on another network and is shot in a different country. (The Flash shoots in Vancouver, while Supergirl films in Los Angeles.)

There was also the Superman problem—the Man of Steel has existed for many years in Kara’s world, but has never been mentioned in the world of Arrow and The Flash.

“We always said very openly that Flash and Arrow exist in a universe where there’s no Superman,” says Berlanti. “Otherwise, you would have heard about him. They would have at least had one conversation about him. That was one thing and then there was the notion that it’s two separate networks. It’s just more challenging in that way. It’s not so convenient. It’s two separate cities that they shoot in.

“We thought we might wait for the second season, but after Christmas we came back and were planning out the back half of the year and we saw a way that we could do it. It felt like the right time for him to enter her world and help her out, so it all came together. Everybody was very enthusiastic. I think one of the things that really kept the flame for it alive was journalists and the fans online and their feedback and enthusiasm for the idea all throughout the year.”

Those fans will be happy to hear that the time and effort was well worth it. Tonight’s Supergirl/Flash crossover event is everything you want an event like this to be. It’s fun, thrilling and laugh-out-loud funny, while also moving Kara’s story forward in a powerful way. The chemistry between Benoist and Gustin is lively and appealing. (We suspect this will be the Supergirl/Flash crossover that launches a thousand “ships.”) While Flash viewers who don’t watch Supergirl will have no problem enjoying the episode, it’s particularly rewarding to Supergirl regulars to see Kara get an assist from another beloved DC super hero right when she needs it the most.

“I think one of the things that I was most gratified about and part of the reason I got so excited about doing this now is that it wasn’t just a gimmick episode,” says Kreisberg. “If you’ve been watching Supergirl, you know that she really needed him this week because of the things that she’s been dealing with.”

Of course, there’s also the matter of the villains, and the episode certainly doesn’t disappoint in that regard, introducing Silver Banshee, a classic Metropolis baddie that has been teased over the past few weeks.

“We’ve been setting up Silver Banshee for a bunch of episodes,” Kreisberg elaborates. “That was a lot of fun to be able to bring on Italia Ricci and establish her as Siobhan Smythe. Our shows tend to be so much about the heroes that the villains tend to get short shrift. This was a real opportunity for us to introduce a character and have them stay on the show for a while. You could get to know them and maybe like them a little bit, so that when they did transform, it actually meant something as opposed to being just a crazy thing that happens at the end of act one.”

Siobhan will find herself teaming up with Livewire, the electro-punk villain played by Brit Morgan. Livewire was introduced early on in the season and proves to be a particularly fitting foil for the Flash.

“For our money, we thought that Livewire was one of our most successful villains from early on,” explains Kreisberg. “So we were very much interested in bringing her back. If you’re going to have two heroes, you’re going to want to have multiple villains. An early version of the story actually had Bizarro teaming up with them as well. It was going to be three against two, but it proved to be financially irresponsible. So we stuck with the two!”

So will “Worlds Finest” be the last time Kara and Barry’s paths cross? Could the Emerald Archer be the next hero to find himself in National City? And does the door swing in reverse? Could Kara ever find herself in The Flash’s Central City or Arrow’s Star City?

“We would love that,” assures Berlanti. “Just like we love this. We’re excited by this. I think we want to wait and see the audience reaction, but just as storytellers and as fans of the actors, we love it when we can figure out a way creatively to . Hopefully everyone enjoys it.”

“Worlds Finest” airs tonight at 8 p.m. (7 p.m. CST) on CBS.