The last time I saw Grant Gustin and Melissa Benoist together is when they appeared as presenters at the Golden Globes, looking like little kids compared to all of the older movies stars and TV actors surrounding them. The next time I see them together will probably be March 28th because that’s when [wait for it] Supergirl will air its official Flash cross-over episode.
After months of rumors and hasty denials, this is actually happening. Greg Berlanti/Andrew Kreisberg’s only superhero show which doesn’t air on The CW will now kind of become part of the Arrow-verse, a cross-over likely made possible since CBS co-owns The CW. Here’s what Berlanti and Kreisberg told the press:
“We are so incredibly excited to announce something that we have dreamed of happening since we starting making Supergirl — The Flash and Supergirl are teaming up! We want to thank Grant Gustin for making the time to come visit (on top of his already immense workload) and all of the folks at CBS, The CW, Warner Bros. and DC for working this out. And finally thanks to the fans and journalists who have kept asking for this to happen. It is our pleasure and hope to create an episode worthy of everyone’s enthusiasm and support.”
There are pretty much no further details beyond that. Don’t ask me what specifically will draw Barry Allen to National City, and whether or not it will just be him (although Kreisberg/Berlanti didn’t mention anyone else from Flash).
If you believe Bleeding Cool, this isn’t an awesome thing happening for purely geeky reasons but actually something CBS requested in an effort to boost Supergirl’s ratings, but why would you let Bleeding Cool bring you down like that? Can’t we just marvel that in the same TV season which gave us John Constantine on Arrow we’ll get The Flash hanging out with Supergirl? Those types of cross-network crossovers would have once seemed impossible.
Now that this is official how do you think they’ll pull it off?
1. It’s An Alternate Universe Version of The Flash
The current season of The Flash has boldly kicked the multiverse door open, presenting us with a new big bad and several new allies hailing from an alternate universe known as Earth-2. In fact, they’ve discovered that there are countless different versions of Earth out there in the multiverse.
In other words, they’ve gone full-on, hardcore comic book on us.
Who’s to say that the Flash/Supergirl crossover couldn’t simply posit the idea that Supergirl takes place in one of the alternate universes? After all, Flash always finds little ways to acknowledge what’s going on its sister show Arrow, but it’s never referenced any of the craziness going down in National City. If they all existed in the same universe, don’t you think Cisco and Caitlin would be going crazy talking about all of the alien activity in National City? We’d probably also be seeing plenty of little easter eggs, like maybe Maxwell Lord’s name on the sides of trucks and buildings ala all of those Kord Industries signs on Arrow.
On top of all that, on Supergirl the Man of Steel has already been an active superhero in Metropolis for years, well before Kara put on her own “S” crest, cape and skirt. That would mean Oliver Queen and Barry Allen were growing up in a world in which Superman existed, yet they never referenced it, even back when Barry was desperate to prove that the amazing was possible as a way of verifying that his story about a red and yellow blur killing his mother wasn’t crazy.
Of course, we know all of those discrepancies exist because Arrow and Flash predate Supergirl, and Berlanti and Kreisberg have only just now received the legal permission to make any kind of crossover. However, to honor the internal logic of the thus-far separate universes being created by the shows the cleanest option for a cross-over would be for the National City universe to have its own version of The Flash.
2. Our Barry Simply Visits An Alternate Universe
Then again, everything I said above would still be satisfied if they simply had our beloved version of Barry visiting what to him is an alternate universe version of National City since back where he’s from there is no such thing as Supergirl or Superman. He could be chasing a metahuman originally from his universe.
3. Don’t Overthink It…
This cross-over will take place on Supergirl and not The Flash meaning my prior two suggestions might not apply because they would require too much explanation. Maybe they’ll just not overthink it and have Barry pop on over for a couple of scenes to help Supergirl and pals battle a metahuman. An obvious model would be his first ever appearance on Arrow when he visited Starling City as a forensics expert from a neighboring police force except in this case he would be called in by Hank Henshaw’s government group.
However, I picture this as being the type of thing where Supergirl might meet The Flash but never find out he’s actually Barry Allen. In fact, I can’t shake the image of Supergirl being on the losing end of a fight only for The Flash to show up out of nowhere to land one of his supersonic punches on the bad guy, turning to Supergirl and flashing his big grin before announcing, “Hi, I’m the Flash.”
Regardless of how this is all engineered and whether or not we’ll get to see Kara and Barry being all adorkable together or if it’ll just be Supergirl and the Flash there is one thing they might not be able to resist: 
These two will have to race to see who’s really the fastest person in the world, right? Plus, since they’re being played by two former Glee cast members they might break out into song, but I’m thinking the race thing seems more likely.
