Siobhan wakes up in the DEO where they analyze exactly what happened to her. Supergirl states that she’s not Kryptonian, and she doesn’t recognize her from Fort Rozz, though it was a big place. Meanwhile, Supergirl has still had problems winning back the city’s trust, down to organizing a family’s Ikea table. A doctor reveals that Siobhan is not an alien but merely emitted a super-sonic scream that pulverized the cement. However, neither can explain where her powers emerged. Siobhan is furious that no one can help her, but Winn insists that no one will abandon her. Suddenly, she experiences a vision of a strange woman and states that she has to go. Before she leaves, she spots Lucy Lane interrogating a captured Livewire on recent electrical surges in the city. Livewire declares her still-strong desire to take down Cat Grant.
At CatCo, Cat lays out a box of cupcakes which Kara eyes deliciously. Cat states that she knows Kara wants one but knows that she won’t do anything about it until they are swept away by somebody else. The conversation then shifts more literally to Kara’s non-existing romantic relationship with James. While Kara’s less optimistic about the possibility, Cat implies that the best way for Kara to win James back is to show disinterest, make him come to her. Kara still states that she’d rather not talk about it but admits that James was the one who wanted time.
Siobhan walks through the city where she keeps having visions and hearing a high-pitched scream. She then marches into CatCo, despite her previous ban, and stares down Cat and Kara for ruining her life. She lets out another ultra-scream which breaks windows and sends Kara flying out the window. When Kara’s falling down the building, a rip in the universe sends a streak bursting through the city that rescues Kara and brings her to the outer desert. The streak is revealed to be none other than The Flash who’s at once shocked that Kara’s not reacting to her clothes on fire. Kara instead removes her clothes and flies back to the city as Supergirl while the Flash runs behind her. When they meet up, he questions how he can fly. When she responds she’s Supergirl, he states he’s never heard of her. She then asks him about what he is where he introduces himself as the Flash, the fastest man alive, whom she’s never heard of before now. When The Flash learns that she has also never heard of Green Arrow, Black Canary, Firestorm, the ATOM, or Zoom, he realizes what’s going on now. He introduces himself as Barry Allen.
Barry: How'd you do that?
Kara: I'm Supergirl?
Barry: You're who now?
At CatCo, Barry researches and finds that while there is a Central City, there is no STAR Labs, Harrison Wells, Cisco Ramon, or Caitlin Snow, no one who can help him return home. Kara’s more curious to learn about Barry’s statement that he is from another earth. She then introduces him to Winn and James, the former who is more than excited at the concept of the multiverse being true. Meanwhile, Barry’s surprised to learn that Kara is an alien. He then explains the concept of the different earths which vibrate at a different frequency with alternating realities, proving if one could move fast enough and vibrate at the same frequency, he could travel to different universes. When James questions how someone could travel that fast, Barry demonstrates by speeding through and giving them all ice cream cones. He explains his story as another superhero, but while has traveled through time before accidentally, he has never breached universes. Until he can figure this out, he’s stuck. Kara says they will help him and proceeds to start by giving him his 10,000 calorie intake. In the meantime, Winn notices James’ discomfort with Barry’s appearance.
Before they can leave, however, they come across Cat who is eager to create a new story about Kara’s rescue by a new superhero rival for Supergirl. When Kara and Barry both try to reason out of that title for the Flash, Cat questions who he is, leading to everyone bumbling through an alibi. Cat then requests James for a photo of the speedster with Kara speaking to no one about the story until they break it. She then tries to come up with a name for the hero, first starting off with The Whoosh or The Red Streak or The Blur. Barry throws in The Flash, but Cat rejects it as a name for a guy who’s only power is jumping out of alleys in a trench coat and settles with The Blur. Cat then talks to Kara and also remarks on James’ discomfort with Barry.
Siobhan meets up with an eccentric aunt as she pawns off items to gullible customers. Her aunt explains to her that their family has been cursed with a banshee who will haunt her for the rest of her life. However, since she has been wronged in her life, she can tap into the power of the spirit with her super-sonic scream. The only way to quiet the spirit is to kill the person who wronged her, though her aunt warns that she will lose her humanity should she do so. Siobhan desires to kill Kara for getting her fired but believes that she is protected by Supergirl and Cat Grant. She decides to break Livewire out of the DEO with her scream, allowing the villain to escape.
Lucy informs Kara that Livewire has escaped, so Kara immediately informs Cat. Kara believes that Livewire will eventually take her revenge on Cat, but Cat stays where she is, stating that unlike the rest of the city, she has faith in Supergirl. With the threat alive and Siobhan still loose, Kara turns to Barry for help. She promises that once those two are caught they will resume their plan of getting Barry home.
Kara introduces that Flash to the DEO where he excitedly tries to take a picture with Kara’s spaceship. Lucy is at first furious with Supergirl bringing a powered being in the premises, but Supergirl states that he is here to stop Livewire with the Flash introducing himself as Barry Allen. He also states that he can help as a CSI forensic scientist, his day job. Supergirl brings him to her sister’s lab, something that is currently vacant after Alex’s absence.
Livewire appears in an abandoned warehouse where there’s case filled with her favorite beer. Siobhan introduces herself, but Livewire’s unimpressed. Siobhan holds her ground revealing that she was the one who broke her out of the DEO and wants her help. Since Livewire wants revenge on Cat Grant and Siobhan wants revenge on her assistant, Siobhan deduces that if they work together, they can take out both women and defeat Supergirl who protects them both. Livewire then agrees to the teamup, as long as Siobhan can change her look.
Barry and Supergirl work on tracking Livewire while James looks on in jealousy. Lucy remarks on that to James who tries to brush it off, but Lucy knows better. She then tells him that if he’s holding back because of her that he has to move on since they were never meant to work out together. Barry finds Livewire in her abandoned warehouse, and Supergirl prepares to rush out to them. Barry, however, states that they should probably have a plan rather than just popping in and fighting, but Supergirl insists that the longer they wait, the worse things will become. The two heroes then take off for the warehouse.
Supergirl and the Flash arrive at the warehouse, which Supergirl proudly states she got there first. The Flash, however, states that he was securing the block. They face off against Livewire with the Flash attempting to use his lighting toss against her, only to help recharge her and have her blast him. Supergirl attempts to set off the sprinkler system but is caught off guard by a sonic scream. Livewire introduces them to Silver Banshee, Siobhan’s new alter-ego. Supergirl tries to reason with her, but she blasts another scream. Supergirl and the Flash use their abilities to blast the villains aside but end up retreating with no other plan.
Back at CatCo, Kara tries to recover but feels loss at how to deal with everything. She apologizes to Barry for rushing in without a plan and explains her sadness at losing the trust of the city. Barry then recounts a time where he also lost the trust of his city but eventually regained it with time. Ironically, he says the best thing she can do is slow down and let the solution happen. Kara thanks him for the advice. The two head back to the DEO.
Cat remains at CatCo despite the threat of Livewire around, but Livewire then appears on the monitors before appearing in the flesh. Silver Banshee then appears to face down Cat about to blast her through the window. Winn tries to reason with her, understanding what it is to have family he isn’t proud of, but she blasts him as well. At the DEO, Barry helps recreate a pair of earbuds he used to defeat a villain named the Pied Piper to block out Silver Banshee’s eardrum-shattering scream. Lucy then informs the two that Cat has been kidnapped, only to be called out by Livewire on screen. The two then race off to face the villains at National City Park.
Barry: What’s say we step away from the nice lady. Settle this like women. What? There are more of you guys here than me.
Livewire and Silver Banshee hold Cat Grant hostage at the city center terrorizing the people to goad Supergirl into appearing. When Livewire’s prepared to kill Cat, Cat finally begs for her life, if only for the sake of her sons. Without her, they would have no one. Supergirl and the Flash then appear to resume their battle with the villains. When Silver Banshee tries her scream, Barry’s earbuds protect the two from shattered ear drums. The superheroes take the fight to the villains with the Flash fighting Livewire and Supergirl stopping Silver Banshee. Supergirl breaks Cat free but is slammed to the ground by Silver a surprise attack Banshee. The Flash tries to fight Livewire, but she escapes through a powerline. When she reappears by Silver Banshee, Livewire spots a helicopter and attempts to take it down. Supergirl then bravely flies in front of the helicopter and takes the full assault of the electrical attack, bringing her down to the ground. Moved by Supergirl’s selflessness, the people of National City stand to protect Supergirl from the villains, even if it risk themselves. When Livewire prepares to blast the citizens, she is blasted by an intense spray of water from a group of firemen, the same ones Supergirl saved before her red kryptonite encounter. They proudly help her stand, stating that it was their turn to save her.
After the takedown, the people of National City have renewed sense of trust of Supergirl, much to Kara’s delight. She then reveals that thanks to Barry, the National City police has a new method of locking up metahumans where they will receive the fair trial they deserve. Kara then asks Cat for some time to see Barry off which Cat allows given his time zipping around in red costume. Kara is shocked that Cat knew all this time, but Cat points out that Barry showed up the same time, insisted on the name The Flash, and portrayed a too up-beat demeanor that he just had to be a superhero. After all, she can spot the extraordinary pretending to ordinary easily.
In the desert, Barry reveals his plan which bares resemblance to his foray into time travel with the Reverse Flash. He believes that with Kara and his speed combined, she can give him the push to finally break the dimensional barrier. In other words, Kara notes, they need to have a race. Before they start, however, Barry gives her a last piece of advice on how to deal with James. While he previously advised to take things slow on being a hero, he states that perhaps she should take things faster in relationships since he figures from experience. The two then take their positions and race through the area before Kara gives Barry a push through the next dimension.
Barry: Think you can keep up, Girl of Steel?
Kara: Just you watch, Scarlet Speedster.
Back at Kara’s apartment, Kara talks to James about what Barry talked to her about different universe, more specifically how they can coexist despite vibrating at different wavelengths. Through the metaphor, she actually states that she believes that they can coexist together, that she’s ready to try this relationship. She punctuates this moment with a passionate kiss to James which he seems to reciprocate. When she’s finished, however, he just stares at her. At first, Kara fears she did something wrong, but then James abruptly walks out of her apartment without a word. Kara then looks out her door and window and sees that everyone in National City is wordlessly walking to the center like mindless drones.
In the distance, it is revealed that Non is responsible for the situation, proclaiming that Myriad has been activated. First, they will take over National City, then the world, for Astra.
- The title "Worlds' Finest" refers to the popular comic-book run of adventures featuring a cross between Batman and Superman called "World's Finest." Obviously, the slight tweak refers to Supergirl and the Flash being from two different worlds (and networks).
- Lots of references to the Arrowverse in this episode
including but not limited to Green Arrow, Black Canary, Firestorm, the ATOM,
Zoom, Central City, STAR Labs, Harrison Wells, Cisco Ramon, Caitlin Snow, Blackout,
the Pied Piper, and Speedy (the name Lucy Lane gives Barry before he introduces
- Barry's reference to an earth where all the good guys are bad guys comes from the episode "Welcome to Earth-2" where he faces off against Killer Frost, Deathstorm, and Reverb, evil counterparts of Caitlin, Ronnie, and Cisco.
- Cat refers to Barry with Supergirl’s team as "the attractive yet non-threatening, racially-diverse cast of a CW show" without really knowing that Barry is indeed from a CW show.
- The Blur might be a familiar term to some DC/CW fans. It’s the same name Clark Kent used for his early superhero days before Superman in Smallville.
- The device on the Flash's costume looks pretty similar to the Reverse Flash's tachyon particle device from the previous season. Could Barry be using it to go faster than Zoom?
- Who do you think would win in a race: Supergirl or the Flash?