For the last few episodes of the CW series, The Flash, Barry Allen, played by Grant Gustin, and his companions have been looking into the identity of Dr. Wells, also known as Eobard Thawne, played by Tom Cavanaugh.

Now that the truth was revealed in episode 20, it is likely that there will be a Flash versus Reverse Flash battle, towards the end of Season 1.

Episode 21 is called "Grodd Lives," and according to the episode synopsis, Barry will be dealing with a new threat caused by the Reverse Flash, but Wells will be using a secret weapon, which is Gorilla Grodd, played by David Sobolov.

Gorilla Grodd last made an appearance in Episode 14 of the show, which was called "Fallout".

In that episode, Reverse Flash left General Wade (portrayed by Clancy Brown) in a sewer for the angry primate, Gorilla Grodd, to finish him. This happened after WAde attempted to kidnap Barry and Firestorm (Robbie Amell) in order to use their powers to help the United States military.

Gorilla Grodd will be returning in episode 21, but Wells will be using him as a weapon against Barry and the STAR Labs team this time. He will not be protecting them as he did previously.

The promotional video for the episode shows that Barry Caitlin, played by Danielle Panabaker, and Cisco, portrayed by Carlos Valdes, will be looking for Grodd in the sewers and this will end in a very frightening showdown.

The synopsis also reveals that Barry and Candice Patton's Iris will be having a serious talk.