(WARNING MAY CONTAIN SPOILERS FOR THE FOLLOWING SHOW'S)
The Flash appears
Yep that's right, the episode is called " worlds finest" ( worlds referring to multiple worlds, the flash's and supergirl')s. Some may have been wondering, how in the hell do supergirl and flash crossover if they don't exist on the same earth ?.Well in an attempt to cross to earth 2 flash ends up on the wrong earth (supergirl's earth).
The Bad guy's
Look in the background, and what do you see. If your answer is Silver banshee then your right, however its gonna take more then her to bring these heroes down. So in this episode she brings livewire to help bring down the girl of steel and scarlet speedster.
In order for his help, his gonna need her's. In exchange for her help to get back to his earth, He will have to help her take down, the bad guy's.
In the comic books the man of steel and the flash face off in a race to see who is the fastest, so to impress the fans why not have flash go against super girl.
SUMMING THIS UP
Super girl and the flash crossover tomorrow and what makes this so big is the fact his on another network and its rare that networks work together. This is defiantly for the fan's.