Arrow and The Flash expanded their shared DC television universe during the first halves of their fourth and second seasons (respectively), in the process laying a foundation for The CW's spinoff series, DC's Legends of Tomorrow. The latter revolves around a team of super-powered individuals - all of whom have previously been introduced on Arrow and/or The Flash - who assemble for a mission to stop the villainous Vandal Savage (Casper Crump): the character who debuted during the Arrow/Flash crossover episodes, "Legends of Today" and "Legends of Yesterday".
Legends of Tomorrow's premiere episode was penned by Phil Klemmer (who will also be serving as the showrunner) as well as the co-creators Greg Berlanti, Marc Guggenheim, and Andrew Kreisberg, while directing duties were handled by Glen Winter (who previously helmed the Supergirl pilot and a handful of Arrow and The Flash episodes). The CW will further promote the series with a half-hour special titled DC's Legends of Tomorrow: Their Time Is Now two days before the first episode airs; in the meantime, the network has unveiled an official synopsis for the Legends premiere.
Here is the official Legends of Tomorrow premiere episode synopsis released by The CW:
In the year 2166, the immortal villain Vandal Savage (Casper Crump) is on the verge of his final victory – total chaos and the utter destruction of humanity. As the world crumbles, the Time Master Rip Hunter (Arthur Darvill) takes matters into his own hands; he travels 150 years into the past to assemble a carefully selected team of heroes and rogues to stop him. Hunter has chosen what seems like an ill-matched group: billionaire inventor Ray Palmer (Brandon Routh), who has created an exo-suit with the power to shrink him to miniscule size, as the Atom; Sara Lance (Caity Lotz), the White Canary, a trained assassin; Professor Martin Stein (Victor Garber) and Jefferson “Jax” Jackson (Franz Drameh), who together form the meta-human Firestorm; Leonard Snart (Wentworth Miller), known as Captain Cold, and his partner Mick Rory (Dominic Purcell), AKA Heat Wave, a pair of career criminals; and Kendra Saunders (Ciara Renée) and Carter Hall (Falk Hentschel) AKA Hawkgirl and Hawkman. Together this unlikely group will attempt to stop one of the most formidable villains of all time, while they must learn how to not only be a team, but heroes as well.
The time-travel setup and related plot mechanics for Legends of Tomorrow will pave the way for the eponymous team of superheroes to not only encounter characters who are brand new to The CW's TV universe, but also cross paths with familiar faces from their past and/or present - including the villains Ra's al Ghul (Matt Nable) and Damien Darhk (Neal McDonough), according to previous reports on that matter. Of course, the show will have to manage the juggling act of balancing the larger plot involving Vandal Savage with story threads for the individual members of the Legends roster; meaning, it would probably be best for the TV series to avoid including too many "special appearances" of that variety.
Legends of Tomorrow is also being designed as an anthology show - where each season is "its own separate movie," as Guggenheim has put it - so the show's main challenge will be to create a dynamic among its team members that's compelling (and entertaining) enough that viewers will want to watch more of their adventures together, regardless of what threats and villains they're facing. The relatively lighter tone of the Legends of Tomorrow footage that's been unveiled to date suggests there's potential for this show to appeal more to the crowd who prefers The Flash over Arrow, but again - that'll depend in no small part on how much fans take to how the Legends themselves work together.
The Flash will return on Tuesday, January 19th, 2016, while Arrow will return on Wednesday, January 20th, 2016 and Legends of Tomorrow will see its series premiere on Thursday, January 21st, 2016.
Source: The CW