Asked how Supergirl will be a part of the series, given her series takes place in another universe, Guggenheim replied, “I think I can say it… I don't think it's that shocking. Last year, Supergirl established that Flash was able to make his way to what I call ‘Earth-CBS’ and it stands to reason that with the proper breach technology the reverse can happen.”
It’s been previously revealed that the villains in the crossover are the aliens known as the Dominators and Guggenheim remarked, “Once the heroes realize that they're up against aliens, they decide that they need an alien on their side. Fortunately, Barry knows a really nice one.”
Guggenheim added, “I don't think it's a big shock that between Barry and Cisco and all their experiences with Earth-2 and the multiverse that they can pluck her from Earth-CBS.”
Guggenheim kept referring to the “three-part crossover,” even though it involved four series and explained, “There’s going to be an episode of Supergirl that ends with Supergirl going over to the crossover. So some people call it a four-way crossover because it involves four shows. My ulcer requires me to call it a three-part crossover. I mean, the story being told has a beginning middle and end. It's a beginning in Flash, a middle in Arrow, and an end in Legends. But Supergirl is very much a part of the whole thing, so we are crossing over four shows in three parts. How does that sound?”
Guggenheim acknowledged that technically, the crossover does at least begin on Supergirl that week, but stressed, “It's right at the end where Barry and Cisco come in.”