Though a success in its own right, The Flash was originally launched as a spinoff from Arrow, with its central character, Barry Allen, showing up alongside Oliver Queen (Stephen Amell) in Star City -- which was still Starling City -- before heading home to Central City and the particle accelerator explosion that would give him his powers. Since its launch, The Flash has featured two major crossover events with Arrow, the most recent of which saw characters from both shows team up to face Vandal Savage (Casper Crump), the villain that will go on to be featured in The CW's third DC Comics-based television series, Legends of Tomorrow.
However, both The Flash and Arrow have enough to deal with in their home cities, with Barry facing the devastating foe of Zoom and Oliver going up against the mystical powers of Damien Darhk. But, David Ramsey, who plays Oliver's friend John Diggle on Arrow, recently teased a trip for his character to Central City. Now, we have more details about that crossover and when it will occur in The Flash.
Read Kreisberg's full quote:
"The Diggles arrive in Central City to help Barry take on King Shark who has escaped from ARGUS and is hellbent on completing his mission of destroying The Flash."
King Shark first appeared at the end of episode 4 of The Flash season 2, 'The Fury of Firestorm', in which he was sent by Zoom from Earth-2 to kill Barry. Instead, Earth-2 Harrison Wells (Tom Cavanagh) defeated King Shark with a weapon from Mercury Labs, which served as the doppelganger character's introduction to Team Flash. Since then, King Shark has been held by ARGUS. This seems to be a tie to the character's comic book counterpart, who has served as a member of the Suicide Squad - he was even rumored to appear in David Ayer's DC Extended Universe Suicide Squad film.
Of course, this crossover won't be as big of an event as that which spanned two episodes, one of Arrow and one of The Flash, earlier this season. To that end, we won't be seeing the same kind of large spectacle and action set pieces usually reserved for the shows' yearly crossovers -- though King Shark may be spectacle enough. Additionally, it will be a much smaller crossover in the sense that only two Arrow characters will be making the jump to The Flash.
But, both shows have featured these minor crossovers in the past few years, with The Flash’s Cisco Ramon (Carlos Valdes) appearing in certain episodes of Arrow, or Felicity Smoak (Emily Bett Rickards) making the trip to Central City to visit Barry. So, although the Diggles' trip to The Flash won't be a major crossover event, it's still a nice reminder for viewers that both CW series take place in the same universe and star a cast of characters who are friends that only live a train ride away. Plus, the return of King Shark will likely excite fans who hoped to see more of the villain after his brief appearance earlier in the season.
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.