CBS just unveiled new information about the upcoming crossover episode of Supergirl and The Flash, as reported by Variety.
The upcoming 18th episode of Supergirl, titled World's Finest, will see the Girl Of Steel team up with the Scarlet Speedster to stop not one but two super-villains, and here's the synopsis:
Kara (Melissa Benoist) gain a new ally when the lightning-fast superhero The Flash, aka Barry Allen (Grant Gustin) suddenly appears from an alternate universe. Barry helps Kara battle Silver Banshee, aka Siobhan Smythe (Italia Ricci), and Livewire (Brit Morgan) in exchange for her help in finding a way to return him home.
By the details of the plot, we can officially say that Supergirl comes from an alternative universe that we can already call Earth-3. Thus, there will be no disrupting of continuity in the Arrowverse to be afraid of. It also means that Barry will end-up on Earth-3 while trying to reopen the portal to Earth-2, probably trying to save the man in the iron mask.
With the notion of multiverse established since the introduction of Earth-2, it means that no show that is part of the DC TV Universe is actually off-limits. Arrow, Legends of Tomorrow and even Vixen could potentially crossover with any other DC shows. Personally, I wouldn't spit on a little Crisis On Infinite Earths type of storyline.
Here's CBS first poster of the much-anticipated crossover: