The Flash belongs to the CW Network, while Supergirl is aired on CBS. Crossovers between two different shows on the same network has become quite common these days (The Vampire Diaries-The Originals Crossover, The Flarrow Crossover, The Chicago Crossover and Legends of Tomorrow-Arrow Crossover, to name a few). But pulling off an inter-network crossover is not that easy. In fact, it's almost impossible.
But they have actually managed to pull it off. Although, the CW Network is partly owned by CBS and partly Warner Brothers, CBS and CW are two separate rival networks and no rival networks would want to do crossovers with each other. Kudos to CW and CBS for working it out and for once putting the fans' interest first. But the man who deserves the most credit for this crossover to actually happen is none other than..
Greg Berlanti is the driving force of the movement to bring these two iconic comic book characters on screen together for a crossover, despite all its difficulties. He started off as the executive producer of Arrow and The Flash. And now he's also the executive producer of Supergirl. The shows, although on two different networks, is run by the same group of people. So, only the two networks needed to give their blessings, and the producers and writers would take care of the rest.
And in early February it was announced that we would indeed be having a Supergirl-The Flash Crossover on Episode 18 of Supergirl, titled "World's Finest", set to air on March 28th. A couple of days later, we even got to see Supergirl as part of an Easter Egg filled scene on The Flash's episode titled "Welcome To Earth 2".
Here's everything you need to know about the Crossover:
The poster of the crossover episode was released a week back and it turned out to be a clever homage to a classic comic book cover which had The Flash racing Superman.
"The Flash comes from an alternative universe and aids Kara in her battle against Siobhan and Livewire in exchange for her help getting home."
Which brings us to the question..
How Does Barry End Up In An Alternative Universe?
The Flash is currently on hiatus and will return on Tuesday, 22nd March with the episode titled "Trajectory". The Crossover will be aired on the very next Monday, 28th March. All the breaches to Earth 2 have been closed. And it has been previously confirmed that Supergirl's Universe is not Earth 2. In the last episode of the Flash, we saw Zoom grabbing Jay and taking him back to Earth 2. It comes as no surprise that Team Flash would try their best to re-open a portal to Earth 2. Our guess is probably, at the very end of the next episode of the Flash, we'll see Barry run through a portal made by Team Flash that actually leads to Supergirl's Earth or we'll see The Flash run very fast in order to catch Trajectory and end up travelling through different dimensions ending up National City on Supergirl's Earth. And the story would pick up from there on Supergirl.