- "No matter what I do to stop Grodd, he just keeps finding new ways to attack, new ways to destroy, and I... Iris doesn't think I should, but the only way this ends is if I kill him."
- —Barry Allen to Harry Wells
It acts as the second half of a two-part episode.
|When Grodd and his army of gorillas bring the battle to Earth-1, The Flash and team must find a way to stop them before they destroy Central City. Gypsy returns to join the fight. Meanwhile, Jesse Quick decides she wants to stay with Wally on Earth-1.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Carlos Valdes as Cisco Ramon/Vibe
- Keiynan Lonsdale as Wally West/Kid Flash
- Tom Cavanagh as H.R. Wells and Dr. Harrison "Harry" Wells
- Jesse L. Martin as Detective Joe West
- Violett Beane as Jesse Wells/Jesse Quick
- Jessica Camacho as Cynthia/Gypsy
- Keith David as Solovar (voice)
- David Sobolov as Grodd (voice)
While H.R. is decorating the Speed lab at S.T.A.R. Labs for Valentine's Day, H.R. and Harry banter on brushing their teeth either front to back or in circles. While H.R. sends out Valentine's Day cards, Jesse talks to Wally about revealing the news to her father that she's moving to Earth-1. After locating Harry, Jesse breaks the news to him. After considering her decision, Harry inquires as to whether she loves Wally and he makes her happy. Once Jesse affirms, Harry says he's fine with it, however, cautions the two not to mess it up. While Caitlin and Cisco Ramon are talking, Gypsy appears from a breach and starts shooting energy blasts at everyone. Harry rushes in and takes her out. Tam Flash detains Gypsy in their makeshift prison where they discover that she was mind-controlled by Grodd and he used her to transport him and his army to Earth-1. After figuring out that Grodd probably scrambled their satellites, the team decides to use Cisco to figure out where Grodd will attack. However, since Cisco's vibes of the future are limited to what Barry saw, Harry figures out that he can widen the scope of Cisco's vibes by upgrading his gear. Cisco keeps assisting that the team should let Gypsy out of her cell so she can help them in their fight. The team eventually agrees, but Gypsy declines to help the team and leaves to hunt a fugitive. After receiving his upgraded glasses for Harry, Cisco sees an intersection in a vibe which is where he believes Grodd's attack will be.
Barry, Joe, and S.W.A.T. arrive at the intersection but believe they are too early because they don't see Grodd and his army. However, once Joe turns a weapon on himself, Barry realizes that Grodd is responsible and they are too late. Before the bullet hits Joe, Barry is able to stop it. Barry realizes that the intersection was only a distraction.
While Caitlin is sewing up Joe back at S.T.A.R. Labs, Joe reveals to Team Flash that he doesn't really remember what he saw when Grodd was controlling him. Harry then reveals that on his Earth, there is a method to achieve memory recovery. Joe agrees to attempt the process. A visibly upset Barry starts to leave and Iris follows him. Barry states that he is tired of seeing the people he love be put in harm's way and that he wants to kill Grodd to stop him once and for all. However, Iris convinces him not to think that way and tells him his mercy is what makes him a hero.
Harry's method of memory recovery is successful as Joe was able to draw the person he saw when Grodd was controlling him. Team Flash was eventually able to figure out the person was Air Force General Matthew McNally. They learn he is in charge of Nuclear missiles and after they got their satellites back online, an alarm went off indicating nuclear weapons were about to be fired.
Barry hurries off and he was able to stop the missile launch with seconds to spare. Grodd and his army then descended upon Central City. Barry, Wally, and Jesse begin to engage the army of gorillas. Shortly after Barry, Wally, and Jesse engaged the army, Cisco and Gypsy arrived with Solovar who challenged Grodd. Solovar emerged victorious from the challenge and was about to kill Grodd when Barry stopped him. Barry was able to convince Solovar to spare Grodd and shortly after, Solovar and the gorilla army returned to Gorilla City.
At Barry and Iris's apartment, the two share a sentimental moment before Barry drops down to one knee and proposes to her.
Wally and Jesse are watching a movie at Joe's house. Jesse specifies that she could eat some Big Belly Burger, so Wally dashes off to get some for the two of them. Outside of Big Belly Burger, Wally sees a speeding white light and is shocked to turn around and see Savitar charging towards him.
- This episode takes place around Valentine's Day.
- When Cisco claims that they need Gypsy, she asks if he's trying to "Luke Starkiller" her, referencing the original name of the protagonist, Luke Skywalker, from the original Star Wars trilogy.
- Harry quotes the 1976 film Rocky, claiming about Cisco's Vibe glasses, "It's an improvement". Cisco picks up on this reference, to which Wells acknowledges the actor who spoke it, Burgess Meredith.
- Wells later references Rocky again, exclaiming, "yo Rocky" when Barry is beating a punching bag, a nod to the title character's role as a professional boxer.
- When they are unable to find the identity of Matthew McNally, Cisco jokes that they're calling him "Jason Bourne", referencing the character from the Bourne novels and films.
- H.R. mentions a poet from his Earth, "Alfred Yankovic", to which Iris asks if he means "Weird Al". "Weird Al" Yankovic is a musician who parodies many famous songs.
- The missile launcher has a striking resemblance to the one used by Edward Fyers in 's Season 1 flashbacks on Lian Yu.
- After Grodd is defeated, he is mentioned to have been detained at A.R.G.U.S.. Caitlin states that Grodd can be thrown into the same cage as King Shark for all she cares, since any good in him is long gone. Cisco then states that was a fight he'd pay money to see. This subtly foreshadowed the Season 5 episode, "".
- The movie that Wally and Jesse are watching at the end of the episode is Casablanca.
- This is the second episode of in which Grodd tries to make Joe shoot himself, the first episode being "".
- Jesse and Cisco both refer to Harry as "Deadman." Deadman is a character in the DC Comics universe and is a member of Justice League Dark.
- When Cisco and Gypsy talk on Earth-19, it is raining in Gypsy's scenes and dry in Cisco's scenes.
- When Barry is trying to enter the correct five-digit code to stop the missile launch, it is mentioned that there are 90,000 possible different combinations. However, a five-digit code has 100,000 possible different combinations, not 90,000.
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