Elseworlds, Part 1

The episode will be the first of the three-part crossover with the ninth episode of the seventh season of, and the ninth episode of the fourth season of.

Synopsis
When Barry Allen and Oliver Queen wake up one morning and realize they have swapped bodies with each other, the two set off to find out what disturbed the timeline to cause such a shift. However, things quickly go from bad to worse when they present their case to Team Flash and the gang doesn’t believe them. Barry and Oliver realize they need Supergirl’s help and travel to Smallville on Earth-38 where they end up meeting Kara’s cousin, Clark Kent, and intrepid reporter, Lois Lane.

Trivia

 * The episode was previously titled "Elseworlds: Hour One".
 * A major Smallville easter egg appears in this episode where the show's theme song "Save Me" by Remy Zero was played during the introduction of the Earth-38 Smallville, with a farm location used in Smallville also reused. In addition, the Smallville version of Green Arrow's suit appeared in the cold open on the corpse of the Earth-90 Green Arrow.
 * Lois calls Clark "Smallville", referencing to Smallville when Erica Durance's Lois Lane would call Clark the same nickname.
 * The scene when Barry shot Oliver with Arrows is almost an exact replica of the original scene from "Flash vs. Arrow", with Barry and Oliver switching dialogues and roles.
 * This episode marks the first appearance of Superman outside of Supergirl.
 * When Oliver was about to faint, Iris said "Nanites, Courtesy of Ray Palmer", which is the same thing Oliver said to Reverse-Flash in.
 * At one point Ralph says, "something weird's going on, and it ain't even Tuesday". This is a nod to the fact that new episodes of The Flash typically air on Tuesday nights, but that the show swapped time slots with Supergirl for the Elseworlds crossover event.
 * and do not appear nor are they credited. However, Cicada is mentioned.
 * is still credited despite not appearing in the episode.