"Elseworlds" is the fifth Arrowverse crossover event that features episodes of the television series The Flash, Arrow, and Supergirl on The CW.

In the crossover, Green Arrow, Flash, and Supergirl, the former two having had their powers and lives switched, are drawn to Gotham City to confront Dr. John Deegan over his work at Arkham Asylum.

Throughout August and September that year, casting for the crossover—including Ruby Rose as Batwoman, Elizabeth Tulloch as Lois Lane, and the announcement that Tyler Hoechlin would return to portray Superman—was revealed.

Oliver and Barry use each other's abilities to escape and travel to Earth-38 to get help from Kara Danvers and acquaint themselves with her cousin Clark Kent and Lois Lane, at Smallville, Kansas.

During a confrontation with inmate Nora Fries, Barry and Oliver are exposed to fear gas and believe each other to be Malcolm Merlyn and Eobard Thawne, respectively.

to restore reality, where Earth-90's Flash warns them about Mar Novu / Monitor, who is testing worlds for an impending crisis.

[22] Despite not being part of the official crossover, Legends of Tomorrow co-showrunner Keto Shimizu revealed "Legends of To-Meow-Meow", the show's episode airing the week of the crossover, would be "crossing over with ourselves" through the use of alternate time periods,[23] and Adam Tsekhman guest stars as Gary Green in the Supergirl episode.

[25] The story for "Elseworlds" was created by Greg Berlanti, the executive producers and writers from each series, Geoff Johns, and Dries.

[11] At the conclusion of "Elseworlds", it was revealed that the next crossover would be "Crisis on Infinite Earths" in late 2019, adapting the storyline from the comic of the same name.

[11] The same end-credits scene teasing the events of crossover was shown at the end of the episodes "Bunker Hill", "Unmasked", and "What's Past Is Prologue".

Supergirl co-showrunner Robert Rovner said the tease was only supposed to air with Flash's "What's Past Is Prologue"; it was added to the other two episodes to make things easier for viewers because they "are coming to [the crossover] from our separate shows and we wanted something that [connects them together]".

[16] Stephen Amell announced that, unlike previous Arrowverse crossovers, this one would not be as extravagant, but more about "the characters and the story",[30] focusing on Green Arrow, Flash, Supergirl, Superman, and Batwoman.

[31] According to Guggenheim, by focusing only on Green Arrow, Flash, and Supergirl, the writers "really had a chance to tell a story about the troika that we've never really been able to do before".

Legends of Tomorrow was not included in "Elseworlds" to highlight the amount of time Amell, Grant Gustin, and Melissa Benoist could interact.

Supergirl is the only other character who can see the true identities of Barry and Oliver; Benoist said, "She inevitably becomes kind of a middleman because ultimately there's conflict between the two and they start butting heads".

[37] The producers noted Rose's passion in her audition and her "aura of cool, nonchalant, somewhat aloof, mixed with charming and thoughtful and funny that it just kind of all made sense for her to be Kate", according to Dries.

[38] Later that month, it was revealed that Tyler Hoechlin would reprise his role as Kal-El / Clark Kent / Superman in all three episodes and that the producers were planning to introduced Lois Lane in the crossover.

[41] By the end of filming in early November, Bob Frazer was cast as Roger Hayden / Psycho Pirate.

[15] In "Elseworlds", Hoechlin wears an all-black Superman suit while portraying John Deegan's antagonistic version of the character.

The website's critical consensus reads, "With disorienting body swaps, confusingly altered realities, and an awesome Superman-Supergirl team-up, there is a lot to love in this joy-filled Arrowverse crossover episode of The Flash".

The verdict reads, "The body-swap premise at the center of the episode made the kickoff to the crossover a delight, though not without its flaws.

The website's critical consensus reads, "'Elseworlds, Part 2' is a riotous continuation of this body-switching Arrowverse arc, successfully exploiting the chemistry between the television universe's ensemble and slickly introducing Ruby Rose as Batwoman".

The verdict reads, "[Part 2] not only manages to continue the momentum (and hilarity) of the previous episode, it also confidently introduces two elements that could have lasting effects on the Arrowverse as a whole -- Batwoman, and the promise of a potential Crisis (on Infinite Earths?).

Fueled by the easy chemistry of Grant Gustin, Stephen Amell, and Melissa Benoist, [this] is one of the most enjoyable episodes from any of the shows in recent years.

The website's critical consensus reads, "'Elseworlds, Part 3' wraps up the crossover fun in style, tying up enough loose ends to satisfy while providing a few unexpected twists to keep fans on their toes".

The verdict reads, "If not the outright best of the Arrowverse crossovers, Elseworlds finds the most success in blending epic superhero spectacle with strong characterization and personal stakes.

[this episode] wraps up the crossover with plenty of both, capping off a particularly strong character arc for Oliver Queen and making terrific use of Tyler Hoechlin's Superman.