Below you can check out some details from the episode as Entertainment Weekly offered up some tidbits. Promos and preview images for the episode are also included below.
Spoiler warning is in effect.
• The crossover will be referenced on the April 5th episode of The Flash, but only literally for a flash as Barry disappears for a second in his world, but ends up for an entire day in the world of Supergirl.
• Supergirl and The Flash exist in parallel universes where they haven't heard of each other's characters, meaning Supergirl doesn't exist in The Flash universe and vice versa: “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.
• It's said since Arrow and The Flash have never referenced Metropolis or Superman because Superman doesn't exist in the Arrow and Flash universe (probably Batman, too): “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.”
• Barry crosses over into the Supergirl universe through use of the tachyon device he has been creating to help take down Zoom.
• It will be a fun and lighter episode, and The Flash and Supergirl are quick to trust each other.
• Barry helps teach Kara about being a superhero; likewise, helping Supergirl helps Barry gain the confidence he needs to fight Zoom back in Central City.
• Winn thinks the idea of a multiverse is cool; Jimmy Olsen feels as if Barry is a little bit of competition; Barry is intimidated by Cat.
• Live Wire and Silver Banshee are the villains.
• There will be race between Supergirl and The Flash.
"Worlds Finest" episode synopsis:
Kara gains a new ally when the lightning-fast superhero The Flash (Grant Gustin) suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.
The Flash and Supergirl preview promos:
The Flash and Supergirl preview images: