|When Gorilla Grodd attacks Central City, Barry and Team Flash find themselves teaming up with an unexpected ally to defeat him – King Shark. However, when they hit a snag, they bring in Dr. Tanya Lamden to try to reach the man behind the shark, Shay Lamden.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West-Allen
- Danielle Panabaker as Dr. Caitlin Snow/Killer Frost
- Carlos Valdes as Cisco Ramon/Vibe
- Danielle Nicolet as District Attorney Cecile Horton
- Jessica Parker Kennedy as Nora West-Allen/XS
- Tom Cavanagh as Harrison Sherloque Wells
- Jesse L. Martin as Detective Joe West
- Audrey Marie Anderson as Director Lyla Michaels/Harbinger
- Zibby Allen as Dr. Tanya Lamden
- Dan Payne as Shay Lamden/King Shark
- David Sobolov as Gorilla Grodd (voice)
Cisco wraps up the meta-human cure, however, Team Flash battles to discover a metahuman to test it on. Barry suggests testing the cure out on King Shark since he believes that King Shark may not want his powers.
Joe returns from his vacation with Wally. While talking with Iris, he informs her that Cecile has brought him up to speed on Cicada and that he believes Cicada has been able to elude capture with help from someone in the Central City Police Department.
At an A.R.G.U.S. aquarium, Team Flash is brought to Dr. Tanya Lamden, who is the ichthyologist working with King Shark. She is able to convince King Shark to come to the surface so Team Flash can talk to him. Shortly after King Shark surfaces, he suddenly starts acting violent out and swims away.
Team Flash and Tanya are eventually able to track down King Shark. Before the team is able to inject him with the meta-human cure, King Shark grabs Cisco. Barry acts quickly and injects King Shark with the cure. The cure works almost immediately and King Shark turns back into Earth-2 Shay Lamden.
Iris confides in Joe that she is afraid to go back to her office after she found out that Cicada knows who she is. Joe then tells Iris to come with him.
After running more tests on Shay, it is uncovered that he experienced a brain wave disruption which is why he had no control over his emotions and judgment. While apologizing to Caitlin and Cisco, Barry is attacked by Killer Frost. After performing a scan on Cisco and Caitlin's brains, it is revealed that Grodd was controlling them and is responsible for Killer Frost attacking Barry. Lyla shows Team Flash the security footage that reveals that Grodd escaped custody on the night of the Enlightenment. The team then realizes that with the telepathic crown, Grodd would be able to control the whole city.
Joe took Iris to a gym to train so she would be ready to fight Cicada if she encounters him again. While there, he reveals to her that he didn't just go to Tibet to see Wally, he also went there to heal.
Barry and Nora run off to stop Grodd from mind-controlling everyone in the city. However, with the telepathic crown, Grodd is easily able to overpower them. Seeing them in danger and wanting to help, Shay convinces Cisco, Caitlin, and Tanya to turn him back into King Shark so he can stop Grodd.
King Shark jumps into battle against Grodd. At first, Grodd gains the upper hand by hanging King Shark upside down, rendering him immobile. Seeing this, Barry and Nora threw lightning at King Shark, waking him back up. King Shark then defeats Grodd.
Grodd is once again placed in A.R.G.U.S. custody and in order to prevent him from escaping again, Caitlin and A.R.G.U.S. medical staff put him into a medically-induced coma. Barry apologizes to Cisco and Caitlin, who accept his apology. Now knowing that the meta-human cure works, Barry suggests that they offer it to Cicada.
- An upcoming fight between King Shark and Grodd was first referenced by Nora West-Allen in "".
- This was also subtly foreshadowed in the Season 3 episode ""; after Grodd was defeated and mentioned to have been locked up at A.R.G.U.S., Caitlin stated that he can be thrown into the same cage as King Shark (who's also in A.R.G.U.S. custody) for all she cares since any good in him is long gone. Cisco then stated that that was a fight he'd pay money to see.
- This episode marks the second appearance of Dr. Tanya Lamden (who last appeared in ""), and the returns of Joe West (who last appeared in ""), King Shark (who last appeared in "" on , while his last Arrowverse appearance was "" on ), and Grodd (who last appeared in "" on The Flash, while his last Arrowverse appearance was "" on ).
- Tanya was recast for this episode; in "", she was portrayed by Haley Beauchamp, while here, she's portrayed by Zibby Allen.
- Joe tells Iris that he traveled to Tibet to "heal", making an inside joke since Jesse L. Martin took a leave of absence from the show to recover from a back injury that he had sustained.
- Nora assumes that, as speedsters, she and Barry should be immune to Grodd's telepathic powers (something she learned from the Flash Museum, which Team Flash informs her is not true). This may be a reference to the fact that Barry is immune to telepathy in the comics.
- Lyla readily accepts that Nora is from the future, pointing out that it's not weirder than finding out that her son used to be her daughter in an alternate timeline, something Lyla found out in "".
- Cisco likens King Shark's freaking out when Grodd tries to control the latter to the 1987 film Jaws: The Revenge.
- Shark Week, an annual week-long TV programming block which features shark-based programming, was mentioned. It also was referenced by Leonard Snart in "" (where King Shark also made an appearance).
- After Cisco and Caitlin are verified to no longer be under Grodd's control, Cisco states "there are no more strings on us", referencing the 1940 film Pinocchio.
- Cisco refers to Grodd as the team's "simian Svengali", harking back to a character from the 1895 George du Maurier novel Trilby.
- Grodd falling from the construction crane and onto a car is a homage to the 1933 film King Kong, in which the titular gorilla, after getting shot by biplanes, falls to his death from the top of the Empire State Building.
- After the battle between King Shark and Grodd, Cisco states that it was the best "Kaijū" battle ever watched, a reference to the popular term used to refer to giant monster characters such as King Kong and Godzilla.
- Before they fight, King Shark says to Grodd, "King Shark wants you GONE!". That is similar to what he said to Barry in season 2 episode "", where he told Barry "Zoom wants you dead!".
- Joe texts Iris saying, "but Grodd hates bananas", which is a reference to the season 1 episode "" wherein Grodd kidnaped Joe and Joe tried to give him a banana, which Grodd refused, as he detests them.
- When King Shark is approaching the dock for the first time, the "Jaws theme" can be heard.
- This is the third episode of The Flash in which Lyla appears without Oliver Queen and the second in which she appears without John Diggle.
- Barry mentions to Nora that he will be doing an "old trick" to try to stop Grodd, that of running in a circle around Grodd before delivering a supersonic punch to him. This is similar to the way Barry defeated him in the season 1 episode "Grodd Lives".
- When Barry and Nora are both paralyzed, Barry says "I can't move!" but then both he and Nora are able to turn their heads to watch the battle between Grodd and King Shark.
- When Barry suggests using the meta-human cure on Cicada, Cisco and Caitlin act as if they're only hearing of this idea for the first time and Barry acts as if he only just came up with it, even though he'd previously mentioned to them that he wanted to use the cure on Cicada.
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