The episode opens on Earth-2 where Harry is running through Gorilla City. He's captured. Back on Earth-1, Jesse fills Team Flash in on her father's whereabouts. He was invited to Gorilla City. The nine people who accompanied him on the trip were all found dead. Barry and Cisco agree to head to Earth-2 to rescue him. Caitlin offers to tag along since Grodd's connected with her in the past. After learning about the mission, Julian's worried about Caitlin, but also excited about traveling to the Planet of the Apes. He joins them on the mission. They reach Earth-2 and are captured almost immediately.
Grodd speaks through Harry and tells them there's another gorilla in charge, Solovar. He hates humans and plans to lead an army to Earth-1. Grodd wants Barry to challenge him in the arena. It's a fight to the death. He says it's the only way to protect Central City. Barry agrees. Cisco and Caitlin offer help through his com device, but it seems to be a losing battle. Barry finally gets the upper hand when he borrows Reverse Flash's go-to murder move. Except he stops short of killing Solovar. He tells all the gorillas watching that humans mean them no harm.
Barry finds himself back in the cell. Grodd takes over Harry once again and the truth comes out: he set the whole thing in motion to get Solovar out of his way. He's the one who wants to lead an army to Central City and take over Earth-1. He wants Cisco to open the breach. Cisco asks Caitlin to kill him. Julian warns her that's a terrible idea, but Barry thinks it's what has to happen. But instead of killing Cisco, Caitlin uses her power on Barry until he appears dead. Once Grodd discovers the body and drags him from the cell, Barry frees everyone else. They speed away, Cisco opens a portal, and they head back to Earth-1.
While they were gone, Jesse and Wally reunited. Jesse was upset Wally hasn't been in contact. She worries he only liked her for her powers and now that he has his own, he doesn't need her. Wally assures her that's not the case. He asks her to stay. She doesn't think she can do that to her father. H.R. steps in and suggests that love is more important than anything else. Once Harry returns, Jesse tells Wally that she wants to stay with him. She plans to tell her father as soon as he's feeling better.
Caitlin doesn't think it's a good idea for Julian to get close to her. She warns him that he should be terrified of her. He agrees to be terrified. But then he suggests they be terrified together over dinner. Caitlin agrees.
Back in Gorilla City, Grodd prepares his army as he orders Gypsy to open the breach for them.