Just like last season with the episodes “Flash vs. Arrow” and “The Brave and the Bold,” Arrow and The Flash will be teaming up again later this fall. While the prospect of seeing the Emerald Archer and the Scarlet Speedster join forces is enough incentive for fans to tune in, this crossover will also help lay the groundwork for Legends of Tomorrow, The CW’s third DC series set in this universe. This will be accomplished not just through introducing new characters, like Hawkman and Vandal Savage, but catching up with old favorites as well.
As executive producer Andrew Kreisberg told THR, the focus of the crossover is still Oliver Queen and Barry Allen having their own adventure, but it will also explain what happened to supporting characters from Arrow and The Flash in setting up how the new heroes and villains will come into play for Legends of Tomorrow. Kreisberg said:
Both Arrow and Flash are helping to set those things up, in both big ways and small ways, which is lovely. The mystery of what happened with Palmer (Brandon Routh) is playing out on Arrow. Sara Lance (Caity Lotz) will also be playing out in the first seven episodes. The Flash, we obviously have Firestorm and Martin Stein (Victor Garber) and Ronnie Raymond (Robbie Amell). And Kendra Saunders (Ciara Renée) lives in Central City in the Flash finale. Getting to know her. There's going to be a lot happening leading up to the crossover, which is serving as a [sort-of] pilot for Legends.
Let’s recap where we left off with each of the existing protagonists back in May. Ray Palmer was tinkering with his A.T.O.M. suit in the Arrow Season 3 finale when there was an unexpected explosion. Obviously he survived somehow, and the Legends of Tomorrow trailer hinted it might have been due to him finally shrinking himself. Before that, Sara Lance was killed in the Arrow Season 3 premiere, but will be revived via the Lazarus Pit in Arrow Season 4 (though she won’t be her old self) and she'll become the White Canary.
Over on The Flash, Martin Stein was last seen with Ronnie Raymond helping the S.T.A.R. Labs team in “Fast Enough,” and Season 2 will explain how Stein will become Firestorm without Ronnie. As for Captain Cold and Heat Wave, the former had his entire record erased in The Flash’s penultimate episode “Rogue Air,” and presumably he and his partner are still on a crime spree across Central City. (Purcell told THR he thinks they're going to "flesh Heat Wave out" in Legends of Tomorrow.)
Jumping to the new protagonists, Kendra Saunders cameoed in The Flash Season 1 finale, and we will see her warrior person Hawkgirl awaken. Aside from a brief mention in The Flash, time traveler Rip Hunter has only been seen in the Legends of Tomorrow trailer, but he will be the one gathering these future “legends" together. Then there’s Jay Jackson, whom we know little about other than he is a former athlete-turned-mechanic that will form a connection to the S.T.A.R. Labs team. All these characters, as well as guest star Hawkman, will be working together to take down Vandal Savage, a 6,000-year-old despot who is determined to dominate the world. That’s a lot to take in for a series, so it’s good that Arrow and The Flash are acting as unofficial prologues.
Fans can look forward to The Flash returning with new episodes on Tuesday, October 6, while Arrow will debut the following day on Wednesday, October 7. Legends of Tomorrow will premiere sometime midseason in 2016.