For the past two years, the big DC TV crossovers have only been between two series, Arrow and The Flash. This year, Legends of Tomorrow and Supergirl are also airing on The CW during the fall, so fans are being treated to a four-show epic story before the year is over. Since none of these shows have begun airing their new seasons yet, we still have a long wait ahead for this crossover, but when it does kick off, Supergirl will be the primary focus.
This past weekend at San Diego Comic-Con (via Comic Book Resources), Arrow executive producer Marc Guggenheim revealed that a powerful force not seen yet on any of the superhero shows will lead Kara Zor-El to go to Earth-1, where all the other DC shows are set. He said:
It's an external threat from outside the shows that will involve Supergirl coming over to The Flash, then Arrow then Legends of Tomorrow.
Marc Guggenheim didn't elaborate on how exactly Supergirl will cross over into Earth-1, but considering how difficult multiversal travel is, this threat must be big enough for her to make the jump to a parallel universe. Unsurprisingly, the order of Supergirl's appearances also lines up exactly with the order these shows will air on the fall. Supergirl retains the same Monday at 8 p.m. timeslot that it did in Season 1. She'll then work her way across the other DC shows each night until the crossover concludes with Legends of Tomorrow on Thursday. It's unclear how many (if any) other heroes will also be crossing over with Supergirl onto a different show, but she'll be the main connective thread. Kara met Barry Allen, a.k.a. Flash, last season in the Supergirl episode "Worlds Finest," so at least she knows who to track down first when she arrives on Earth-1.
One of the popular theories among fans right now is the DC crossover will somehow lead to Supergirl's Earth being merged with Earth-1. That way it would be easier for Supergirl to crossover with the other shows, whether another hero jumps to the Girl of Steel's neck of the woods or she guest stars elsewhere. That has yet to be verified, but fortunately, Supergirl moving from CBS to The CW has already helped in terms of shooting in the same city as the other shows, Vancouver. So even if there isn't a merging by the end of the of the crossover, at least coordinating schedules for other crossovers will be less difficult than when Supergirl was in Los Angeles.