The CW have released the episode synopsis for their three part Elseworlds Arrowverse cross-over…
The Flash: “Elseworlds, Part 1”
When Barry Allen (Grant Gustin) and Oliver Queen (guest star Stephen Amell) 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 (guest star Melissa Benoist) help and travel to Smallville on Earth-38 where they end up meeting Kara’s cousin, Clark Kent (guest star Tyler Hoechlin), and intrepid reporter, Lois Lane (guest star Elizabeth Tulloch). LaMonica Garrett guest stars as The Monitor. Kevin Tancharoen directed the episode written by Eric Wallace & Sam Chalsen.
Arrow: “Elseworlds, Part 2”
With Oliver (Stephen Amell) and Barry (guest star Grant Gustin) still stuck in the other’s bodies, the two get a lead on John Deegan (guest star Jeremy Davies) and head to Gotham City with Supergirl (guest star Melissa Benoist) to figure out why their reality has changed. While there, they meet the mysterious Kate Kane (guest star Ruby Rose) who provides them with information that leads the group to Arkham Asylum. James Bamford directed the episode with teleplay by Marc Guggenheim and story by Caroline Dries.
Supergirl: “Elseworlds, Part 3”
Supergirl (Melissa Benoist), The Flash (guest star Grant Gustin), Green Arrow (guest star Stephen Amell) and Superman (guest star Tyler Hoechlin) engage in the battle of their lives. Jesse Warn directed the episode with story by Marc Guggenheim and teleplay by Derek Simon & Robert Rovner.