The much hyped crossover of Supergirl and The Flash surpasses expectations. The Central City Speedster and The Girl of Steel, names they affectionately call each other, turn out to be a winning formula that should jolt CBS into renewing Supergirl for Season 2. Supergirl started strong, but ratings have dwindled dangerously. Additional episodes were ordered to give the show more time to build an audience, with no commitment from the network for a sophomore season. CBS owns part of The CW network where The Flash is a mammoth hit. Greg Berlanti, producer of both shows, engineered the crossover episode. It is a ratings stunt across networks, but the pleasant surprise is that it works. Resulting in the best episode of Supergirl yet, exactly what was needed to boost the show. There are complete spoilers below, so be forewarned.

Episode 18, World's Finest, begins with disgraced reporter, Siobhan Smythe (Italia Ricci), being examined at the D.E.O. labs. In last week's episode, she fell off a roof, but used the sonic power of her voice to halt her fall. Wynn (Jeremy Jordan) reveals his friendship with Supergirl (Melissa Benoist), but doesn't divulge her secret identity of Kara Danvers, who Siobhan has sworn vengeance upon. They find no clues to her powers. Bitter and angry, Siobhan has a brain seizure, that points her in the direction of Livewire's (Brit Morgan) specialized prison cell. She overhears Livewire being questioned by Lucy Lane (Jenna Dewan-Tatum). Citizens are having brain trauma caused by electrical impulses. Livewire knows nothing, but curses her hatred for Supergirl and Cat Grant (Calista Flockhart). Siobhan stalks out of the D.E.O., seemingly under mind control.

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Back at the CatCo building, Kara's now public love for Jimmy Olsen (Mehcad Brooks) results in unsolicited advice from Cat Grant. Make him jealous, stop pursuing. The elevator opens and Siobhan strolls toward Cat's office. Kara and Wynn implore her to leave, but get a dose of her sonic power. Kara is knocked unconscious and flung out of a window. As she plummets, The Flash (Grant Gustin) appears out of nowhere to rescue her. He zips her to the countryside, but is dumbfounded when Kara sheds her clothes and leaps into the air as Supergirl. He catches up to her, introduces himself as Barry Allen, The Flash from Central City. Kara has no idea who he is, and he has never heard of her. A realization hits Barry like a brick, in his attempt to get faster for his rematch with Zoom; he has broken the dimensional barrier to an alternate Earth.

The concept of the Multiverse is the essential plot thread of The Flash. It works brilliantly here, allowing the superhero crossover to be explained without any issues. There's a great scene where Barry uses a whiteboard to explain how infinite Earths exist in the same space time, but vibrate at different frequencies. Kara and Barry connect on multiple levels. She can talk to him as a peer, and they are both incredibly geeky. The blossoming bond flicks the jealousy switch in Jimmy Olsen. Turning the tables on the budding romance with Kara, leaving Jimmy feeling woefully inadequate to Barry.

While Barry and Kara become besties, Siobhan discovers the source of her power. An aunt tells her of a family curse that becomes active when they are wronged. It is the power of the banshee that will grow and consume her until she kills her sworn enemy. Siobhan wants to kill Kara, but can't face Supergirl alone. She uses her now understood powers to break Livewire out of the D.E.O. prison.

Lucy Lane informs Kara of Livewire's escape. Kara begs Cat Grant to hide, but she refuses to look weak. Barry devises an algorithm to detect power surges. He quickly discovers where Livewire has gone, but is hesitant to face her without more information. Kara disagrees, she fears for Cat and the public's safety. They race to an abandoned warehouse for the face-off. Livewire has transformed Siobhan into Silver Banshee with some Halloween make-up and leather pants. Supergirl and The Flash weren't expecting two baddies. They get a sonic beat down and are forced to retreat.

Supergirl and The Flash head back to the D.E.O. to get better prepared for the second round. Barry, the seasoned crime fighter and technical genius, builds ear buds to neutralize Silver Banshee's powers. Livewire takes control of the airwaves. They have Cat Grant in a city park and will kill her. The second showdown actually doesn't fare much better for our heroes. Livewire zaps The Flash then beats Supergirl as she saves the park citizens from being destroyed by a falling helicopter. In a stunning turn, the citizenry, the little people that always need saving, become the heroes. They shield Supergirl as firemen hose down Livewire. She short circuits and electrocutes Silver Banshee in the process. Honestly, it's a little hokey, but feels right that costumes weren't needed to win this battle. And in the aw shucks moment, Supergirl regains the trust of the people after her poison-induced bad behavior a few episodes back.

World's Finest ends on a high note, but with an enticing teaser. Barry shows the police how to imprison meta-humans, thus getting the D.E.O. out of the prison game. He also tells Kara to shake a leg on her romance with Jimmy and get to it. They hug it out, and then embark on the mother of all races. Barry surmises that if Supergirl can throw him forward at top speed, he should have enough velocity to break the dimensional barrier. The plan works, was there any doubt, as Kara tosses Barry back to his Earth. She then musters enough nerve to invite Jimmy back to her apartment and lay some smooching down. His reaction is unexpected. Jimmy goes blank, then walks like a drone out of her door. Kara watches as all the humans on the street are walking like zombies. The screen then flashes to Non (Chris Vance), her chief adversary and leader of the escaped Kyptonian prisoners. Their diabolical plan that has been alluded to in previous episodes, Project Myriad, has begun.

The Flash and Supergirl crossover ticked every box. It was highly entertaining with a concise plot and excellent pacing. Grant Gustin and Melissa Benoist were fantastic. They nailed exactly what their relationship should be, friendship and teamwork. Normally I find the romantic subplots of Supergirl to be silly and melodramatic. It's done with a great touch here. The idea of Barry nudging Kara and Jimmy together is pulled off with deft aplomb. Even the one-liners from Cat Grant taking a shot at The CW's multi-racial casting were a hoot. Supergirl hasn't been this clever or good, so a tremendous achievement by the production staff. The show just needs to keep up this level of quality and get renewed. Could Supergirl be a factor in helping Barry defeat Zoom? Now that would be a crossover to top this one.