'The Flash' will face a very different kind of villain in Season 4

After three seasons of speedy villains, The Flash will hit the brakes in Season 4.

"Next season we're not going to have a speedster [as the main villain]," showrunner Andrew Kreisberg revealed during a panel at Los Angeles' PaleyFest on Saturday, where the stars and producers of The Flash, Arrow, Supergirl and Legends of Tomorrow were gathered to celebrate the four CW shows.

But before Barry Allen (Grant Gustin) can slow things down, he needs to defeat this season's Big Bad. That would be Savitar, a self-proclaimed speed god, who is threatening to kill Barry's soulmate, Iris West (Candice Patton).

"With Savitar, we know who he is under there," Kreisberg teased. "We've been planning this for a while and we're really excited about it."

According to the showrunner, the opening seasons of Flash were about reveling in having powers, whereas the current episodes are about their pitfalls. "That's the story we wanted to tell and that's why our version of Savitar is the perfect villain for that story," he said.

The March 21 episode of The Flash, "Duets," is a musical crossover with Supergirl, featuring Glee alum Darren Criss as Music Meister, who traps Barry and Kara (Melissa Benoist) in an alternate reality that operates like a movie musical.

Kreisberg said the episode takes a page out of Joss Whedon's playbook, noting that even in Buffy the Vampire Slayer's "gimmick" episodes like "Hush" and "Once More With Feeling," the storylines "didn't stop the ongoing narrative of the season; those episodes really spoke to what the characters were going through.

"We set it up so that in The Flash and Supergirl, Barry and Kara would be at a romantic crossroads…they can go on this journey together to either get back the love they lost or not."

After his sojourn through a few musical numbers, Barry will be trying to figure out just how to stop Savitar, and episode 18 will feature a villain from the future called Abra Kadabra.

"He knows who Savitar is and it becomes a moral conundrum: Do Barry and the team let Abra Kadabra go in order to get Savitar's identity?" Kreisberg explained. "Iris and Barry have a great scene, 'Can you be a hero if you do one bad thing?'"

Episode 19 is called "The Once and Future Flash," in which "Barry decides that the only way he can find out what he needs to know is in the future," Kreisberg said.

Gustin admitted he understands viewers who are frustrated with some of Barry's questionable choices in Season 3 -- including his decision to take a break from his relationship with Iris in episode 316.

"I agree most of the time, as a viewer, with the backlash that Barry receives," he revealed. "He's always been an optimistic character and I think this year we just see his fear dictate things more than ever, because how could it not with what he's going through?"

Supergirl airs Mondays at 8 p.m., The Flash airs Tuesdays at 8 p.m., Arrow airs Wednesdays at 8 p.m. and Legends of Tomorrow airs Thursdays at 8 p.m. on The CW.