The CW has released an official description for the December 8 episode of The Flash.
The episode is called “Running To Stand Still” and here’s how they describe it:
MARK HAMILL, WENTWORTH MILLER AND LIAM MCINTYRE RETURN; KEIYNAN LONSDALE JOINS THE CAST AS WALLY WEST — When Mark Mardon AKA The Weather Wizard (guest star Liam McIntyre) returns to break Leonard Snart AKA Captain Cold (guest star Wentworth Miller) and James Jesse AKA The Trickster (guest star Mark Hamill) out of Iron Heights, Barry (Grant Gustin) must stop these rogues from taking over Central City during Christmas. Meanwhile, Joe (Jesse L. Martin) and Iris (Candice Patton) meet Wally West (Keiynan Lonsdale). Kevin Tancharoen directed the episode written by Andrew Kreisberg (#209). Original airdate 12/8/2015.