Elseworlds, Part 2

The episode is the second 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.
 * Nora takes a freeze ray from glass case that's labelled Fries, V. This is a reference to her husband Victor Fries, or Mr. Freeze, one of Batman's greatest villains.
 * 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 (Penguin), Pamela Isley (Poison Ivy), Basil Karlo (Clayface) and Edward Nigma (The Riddler).
 * 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.
 * 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.
 * 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 Batmanand Batman Returns
 * When Oliver (as the Flash) misaims a lighting bolt, the gas released falls out of a box labelled "Crane, J". This is a reference to Jonathan Crane, a.k.a. the Scarecrow, one of Batman's greatest villains.
 * The Earth-Ninety Barry mistakes John Diggle for John Stewart who in the comics is one of the characters who become Green Lantern.
 * Barry narrates the opening 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.