Warning: spoilers for the latest Supergirl episode are ahead! If you haven't caught the episode, please check out one of our other lovely articles.
Last night, Supergirl delivered the moment many DC fans have been waiting months to see: Kara Zor-El teaming up with Barry Allen, a.k.a. The Flash. After accidentally traveling from Earth-1 to Earth-CBS (seriously, we need an official name for Supergirl’s world), Barry helped Flash battle Livewire and Silver Banshee, and at the end of "Worlds Finest," Kara used her own enhanced speed to help Barry get back home. (This process is similar to the one Barry used to get Eobard Thawne back to his time period in “The Reverse-Flash Returns.”) While the duo joining forces was fun to watch and opens the door for other crossovers between the CBS and CW DC shows in the future, there was one issue that was left unexplained to The Flash fans: how exactly did Barry arrive in Supergirl’s world?
When Flash showed up in Supergirl last night, he rushed out of a blue portal and immediately grabbed Kara, who had been literally screamed out of the CatCo building by Silver Banshee. Now, anyone who’s been watching The Flash Season 2 shouldn’t be surprised by the portal Barry came out of, since they’re the same kind that lead to Earth-2. The problem is that in “Escape from Earth-2,” all the multiversal breaches were closed, and while we know new ones will open up so that Barry can have his showdown with Zoom, as of “Trajectory,” there’s no way out of Earth-1. In other words, Barry arriving in Earth-CBS doesn’t make any sense… yet.
Admittedly, the main reason for the confusion is due to the respective Supergirl and The Flash episodes airing out of order. While chatting with IGN, executive producer Greg Berlanti said that Supergirl’s "Worlds Finest" takes place after tonight’s The Flash installment, "Flash Back." So after that airs, then "Worlds Finest" happens, and then presumably those events will be touched upon in The Flash Episode 18. As for the in-continuity explanation, the answer seems to be the device on his chest.
Barry was wearing that thing over his chest when he arrived and left on Supergirl, but judging by that comment he made about how he was testing his speed, he didn’t expect to end up in another world. Although Barry’s been able to create temporal breaches before with just his speed, the paths between worlds have always been more difficult to create, even when you take into account the singularity from the Season 1 finale. Aside from his speed, costume and civilian clothes, all Barry had on his person in Supergirl was that device. So while he was running in his own world, maybe his speed triggered something in the machinery that transported him to Supergirl, but to get back to Earth-1, he needed all that and Kara’s own speed.
Tonight’s episode of The Flash will see Barry traveling back in time to when he fought Pied Piper in the Season 1 episode "The Sound and the Fury." Barry has traveled back in time before without using any kind of external apparatus, but he hasn’t always had great control over where… sorry, when he ends up. So perhaps that chest device somehow focuses his speed so that he can hone in on a specific destination, like the Cosmic Treadmill in the comics. After making his way back to the present, perhaps Barry and the S.T.A.R. Labs team will adjust it so it becomes a way to open world breaches. Or, maybe when he was traveling back to the present, something strange happened that resulted in him ending up in Earth-CBS. Whatever's happening here, maybe that chest device is the key to getting Flash back to Earth-2 and defeating Zoom once and for all.
We’ll get some answers about Barry’s unusual trip to Supergirl when The Flash airs tonight at 8 p.m. EST on The CW. As for Supergirl, you can follow the final episodes of Season 1 on Mondays at 8 p.m. EST on CBS.