The episode reunites Melissa Benoist with her former Glee recurring co-star Grant Gustin, who portrays the Flash.

The episode title was inspired by the World's Finest Comics series, in which Superman would team up with various other DC superheroes, including the Flash.

the Flash, a superhero from a parallel Earth, after Kara reveals herself as Supergirl, and Barry later learns her extraterrestrial origin.

At CatCo, Barry meets Winn and James and explains the multiverse and how he reached their universe when testing a tachyon device fastened to his suit.

The crowd surrounds her, and firefighters blast Livewire with water, causing her to lose control and shock Silver Banshee, defeating both.

At the field, Barry determines the kinetic energy from their combined running speed, with the tachyon device active, should create a breach back to his Earth.

In November 2014, creator Greg Berlanti expressed interest in Supergirl existing in the Arrowverse, the same universe as his other series Arrow and The Flash,[1][2] and in January 2015, The CW president Mark Pedowitz revealed that he was also open to a crossover between the series and networks (due to Berlanti executive producing all three and The CW being co-owned by CBS).

[8] While no plot details on the episodes were released at the time, Ross A. Lincoln of Deadline Hollywood noted that "the in-universe reason" for the crossover was due to Barry's ability to travel to various dimensions, thus implying that Supergirl exists on an alternate Earth to Arrow and The Flash in a multiverse.

[9] The earth that Supergirl inhabits is Earth-38 in the Arrowverse multiverse,[10] and has been informally referred to as "Earth-CBS" by Marc Guggenheim, one of the creators of Arrow.

Not only did Barry Allen fit perfectly in Kara's world, but actors Grant Gustin and Melissa Benoist had fantastic chemistry together onscreen.

Club gave the episode an A−, stating "My biggest fear going into "World's Finest" was that the episode would be a fun one-off designed to convert Flash fans into Supergirl fans (and vice versa, I suppose) while ignoring the compelling narrative Supergirl has been exploring since Kara went bad in "Falling".

Instead "World's Finest" relies heavily on Supergirl's long-term storytelling, pulling together threads from the entire season into one jam-packed episode.

For better or for worse, "World's Finest" represents Supergirl in a nutshell: Fun, clunky, cheesy, and more intelligent than it seems at first glance.