After teaming up with Oliver Queen once again in this week's Arrow and The Flash two-night crossover event, Barry Allen (Grant Gustin) returns to Central City to take on three returning villains in next week's midseason finale. The CW has released the first trailer for the last episode of 2015, Running to Stand Still, airing Tuesday, December 8 at 8 PM ET on The CW. This finale will surely be explosive, as The Weather Wizard, Captain Cold and The Trickster all make their return, while fans will also get to meet another new character.

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 must stop these rogues from taking over Central City during Christmas. Meanwhile, Joe and Iris meet Wally West (Keiynan Lonsdale). This episode will mark Wally West's debut, before he returns in the back half of this season.

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Keiynan Lonsdale was cast as Wally West back in August, but it was never revealed when this character would debut on The Flash. In the comics, Wally is the nephew of Iris West, played by Candice Patton in the series. He eventually turns into the character known as Kid Flash, after an accident that is reminiscent of how Barry Allen (Grant Gustin) gets his powers. We still don't know if Wally will turn into Kid Flash at any point during this season, but it's certainly possible.

Since it isn't known when The Flash will return from its holiday hiatus, it's possible next week's episode could be Captain Cold's last appearance on The Flash for awhile. The villainous character will be shifting over to the spinoff DC's Legends of Tomorrow, premiering Thursday, January 21 at 8 PM ET on The CW. It's also possible that The Flash could also return that week, along with Arrow, to lead into DC's Legends of Tomorrow.

Other new characters debuting on The Flash this season are Michael Ironside playing Lewis Snart, Shantel VanSanten as Patty Spivot, Violett Beane as Jesse Quick, Demore Barnes as Henry Hewitt and Adam Copeland, a.k.a former WWE Superstar Edge as Atom Smasher. Gorilla Grodd also returned in last month's Gorilla Warfare, but it isn't known if he will be return in the back half of this season. Take a look at the latest photos before the midseason finale airs next week.