You can check out the official synopses below:
“Elseworlds, Part 1” — (8:00-9:00 p.m. ET) (Content Rating TBD) (HDTV)
EPIC ELSEWORLDS CROSSOVER KICKS OFF TONIGHT — 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.
“Elseworlds, Part 2” — (8:00-9:00 p.m. ET) (Content Rating TBD) (HDTV)
THE ELSEWORLDS CROSSOVER CONTINUES IN GOTHAM CITY WITH BATWOMAN (RUBY ROSE) — 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.
“Elseworlds, Part 3” — (8:00-9:00 p.m. ET) (Content Rating TBD) (HDTV)
THE BATTLE CONCLUDES — 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.
Elseworlds stars Grant Gustin as Barry Allen / The Flash, Candice Patton as Iris West-Allen, Danielle Panabaker as Caitlin Snow / Killer Frost, Carlos Valdes as Cisco Ramon / Vibe, Stephen Amell as Oliver Queen / Green Arrow, David Ramsey as John Diggle, Emily Bett Rickards as Felicity Smoak, Melissa Benoist as Kara Zor-El / Kara Danvers / Supergirl, Tyler Hoechlin as Kal-El / Clark Kent / Superman, Ruby Rose as Kate Kane / Batwoman, Elizabeth Tulloch as Lois Lane, Cassandra Jean Amell as Nora Fries, Jeremy Davies as John Deegan, Bob Frazer as Roger Hayden / Psycho-Pirate, LaMonica Garrett as Mar Novu / Monitor, and John Wesley Shipp as his 1990s version of Barry Allen / The Flash.
Arkham Asylum doctor John Deegan rewrites reality, which results in Oliver Queen and Barry Allen swapping lives. In the new reality, they are the only ones that know that they are in the wrong lives and have each other’s powers, which results in them going to Gotham City with Kara Danvers / Supergirl to confront Deegan.
The Elseworlds crossover will debut on December 9th, 10th and 11th at 8 pm ET on The CW.