Episode 20 of “The Flash” proved that the show is about to be a whole new ball game. For the last few weeks on the CW series, Barry Allen (Grant Gustin) and his friends have been investigating the actual identity of Dr. Wells/Eobard Thawne (Tom Cavanaugh). Now that the truth is out in the open, it will be a Flash vs. Reverse Flash battle as Season 1 winds down. So what will happen in episode 21, “Grodd Lives?”

According to the synopsis for the episode, Barry will be tending to a new threat posed by the Reverse Flash. But Wells will be deploying a secret weapon – Gorilla Grodd (David Sobolov).

Fans will remember Gorilla Grodd from his appearance in episode 14, “Fallout.” In that episode, Reverse Flash left General Wade (Clancy Brown) in a sewer to be finished off by the angry primate after Wade tried to kidnap Barry and Firestorm (Robbie Amell) to harness their powers for the U.S. military.

Gorilla Grodd will be back in episode 21, but this time Wells will be using him to attack Barry and the S.T.A.R. Labs team, not protect them. The promo reveals that Barry, Caitlin (Danielle Panabaker), and Cisco (Carlos Valdes) will go looking in the sewers for Grodd in what should prove to be a terrifying showdown.

Watch the promo for episode 21 below:

The synopsis also reveals that Barry and Iris (Candice Patton) will have a serious discussion. After the Flash saved Iris from the Reverse Flash at the end of episode 20, a shock between the superhero and the reporter made Iris recognize Barry as the man behind the mask. Since everyone else in Iris’ life already knows Barry’s secret, it should be an interesting conversation.

Will Barry be able to stop Gorilla Grodd? Can he stop the Reverse Flash? Fans will have to wait to find out. “The Flash” airs Tuesdays at 8 p.m. EDT on the CW.