Executive producers Aaron and Todd Helbing told E! News that episode 304 is an origin story of sorts for Mirror Master, played by Aquarius’ Grey Damon—and it’ll also mark the return of Wentworth Miller as Captain Cold.
“It’s kind of the origin of Mirror Master…We’re really excited,” said Aaron. Added Todd, “It’s the origin story of him, but it’s this struggle between Mirror Master and Captain Cold and you get to see who comes out on top.”
You’ll have to wait until the fall to find out who that is—but you should know that everyone on The Flash was very excited for Miller’s return. “We’re excited to have Wentworth back because he always brings this amazing presence,” said Aaron. Miller is the first DC TV actor to strike a unique deal that allows him to be a series regular on every CW superhero show: Flash, Arrow, Legends of Tomorrow and now Supergirl.
Source: http://www.eonline.com