Elseworlds, Part 2

The episode is the second part of the three-part crossover, ​, with the ninth episode of the fifth season of ​, and the ninth episode of the fourth season of ​.

Synopsis
With Oliver and Barry still stuck in the other’s bodies, the two get a lead on John Deegan and head to Gotham City with Supergirl to figure out why their reality has changed. While there, they meet the mysterious Kate Kane who provides them with information that leads the group to Arkham Asylum.

Trivia

 * The episode was previously titled "Hour Two".
 * Some of the locations in Chicago that were filmed in this episode were the same locations used in The Dark Knight trilogy to replicate Gotham City.
 * This episode marks the very first time in the Arrowverse where Batman is mentioned by name on screen.
 * Bruce Wayne, who was mentioned before in "​"​, is also mentioned several times in the episode.
 * This episode features many references to Batman:
 * The inmate rooms inside of Arkham Asylum contained nods to several Batman villains on each door including: Oswald Cobblepot (The Penguin), Pamela Isley (Poison Ivy), Basil Karlo (Clayface) and Edward Nigma (The Riddler).
 * Despite the references to the Batman villains, there was one name on an inmate's room that instead referenced writer and former ​​ showrunner Marc Guggenheim.
 * The WiFi password Kate gave to Oliver, Barry and Kara is "Alfred", which is a reference to Batman's butler Alfred Pennyworth.
 * Digger Harkness' boomerang, Bane's mask from The Dark Knight Rises, Hugo Strange's sunglasses, the Riddler's cane, the Penguin's umbrella, and other items can be seen inside of Deegan's lab.
 * Barry asks if a ride in the Batmobile is out of the question, referencing Batman's vehicle from the comics.
 * Kara finds a William Shakespeare bust in a box which is a reference to the 1960's  Batman TV series where it has a button to the entrance of the Batcave.
 * When Oliver mis-aims a lighting bolt, the gas released falls out of a box labelled "Crane, J". This is a reference to Jonathan Crane (The Scarecrow), one of Batman's greatest villains.
 * Nora Fries takes a freeze ray from a glass case that's labelled "Fries, V". This is a reference to her husband Victor Fries (Mister Freeze), one of Batman's greatest villains.
 * Victor Fries was mentioned before in "​"​.
 * A Gotham police officer mentions Nolan and Burton streets. This is an homage to Christopher Nolan, who directed the three films in the Dark Knight Trilogy, and Tim Burton, who directed Batman and Batman Returns.
 * When Supergirl says goodbye to Batwoman, she says that they both would have been a good duo, to which Batwoman replies as the "World's Finest". This is a reference to a crossover in the comics between Batman and Superman named World's Finest where they both team up.
 * The Earth-90 Barry points out that the John Diggle of Earth-1 doesn't wear a ring like his Earth-90 counterpart. This is a reference to John Stewart, who in the comics is one of the characters who wear the Green Lantern Ring and becomes Green Lantern.
 * Barry narrates Arrow opening mid-Season 7 introduction (using the name Oliver Queen) due to the altered reality where he's Oliver Queen/Green Arrow.
 * At the end of the episode, the Monitor reveals that he is preparing them for a "Crisis". This ties in with the red skies emerging in the first two parts of the crossover. Both are a reference to the Crisis story arc, which includes Crisis on Infinite Earths, Infinite Crises, and Final Crisis.
 * The scene were Earth-90 Barry Allen was trying to tell them about the book of The Monitor while stuck in the portal might had been a nod to Batman v Superman: Dawn of Justice when future Barry Allen in that film was trying to warn Bruce Wayne in the present.