The Flash
Dr. Wells (Tom Cavanagh, pictured) synthesizes Velocity 6 in episode 9 of "The Flash" Season 2. The CW

The main storyline of the first crossover episode between "The Flash" and "Arrow" Tuesday saw Barry Allen (Grant Gustin) and Oliver Queen (Stephen Amell) teaming up to protect Kendra Saunders (Ciara Renée) -- who later "emerged" as Hawkgirl -- from immortal bad guy Vandal Savage (Casper Crump). However, it is the "B" plot of the episode that may have more lasting ramifications for Barry and the team at S.T.A.R. Labs.

While the Flash and the Green Arrow were off dealing with Savage, Dr. Wells (Tom Cavanagh) and Caitlin (Danielle Panabaker) created a speed-enhancing serum called Velocity 6 to get Barry up to speed to fight Zoom (Tony Todd). Jay Garrick (Teddy Sears) eventually had to use that serum to save Wells' life after Patty Spivot (Shantel VanSanten) showed up spooked to see Wells alive and put a bullet in his chest. However, his one request after saving Wells is that Barry not be given Velocity 6, which he warned did more harm than good. Executive producer Andrew Kreisberg, though, says fans should not expect Wells to listen to Jay.

“If you’re a fan of the comics, you know that a big part of the comics is Velocity 9. And [in Tuesday's episode] you guys met Velocity 6. So it’s safe to say that there’ll be a few more iterations of that formula," said Kreisberg, according to Variety. "How it plays out and how it works and who uses it and the effects that that have -- that’s something that you’ll have to watch.”

In the comics, Velocity 9 is a drug developed by -- wait for it -- Vandal Savage! The compound gives the user super speed, but it can also have disastrous, physical side effects.

In the meantime, the crossover fun continues on "Arrow" as Barry and Oliver continue their fight with Vandal Savage and sort out the whole Hawkman/Hawkgirl problem. Considering Tuesday's "Arrow"-heavy cliffhanger, fans of "The Flash" will probably want to tune in to avoid missing anything important. Part 2 of the crossover event airs Wednesday at 8 p.m. EST on the CW.

Watch the promo for the "Arrow" crossover episode below:

How will Velocity 6 (or 9) factor into the show in the future? Only time will tell. "The Flash" airs Tuesdays at 8 p.m. EST on the CW.

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