- For the eponymous character, see Gwyn Davies.
"It's a Mad, Mad, Mad, Mad Scientist" is the fifth episode of the seventh season of , and the one-hundred-second episode overall. It aired on November 10, 2021.
|After Sara, Ava, Behrad and Gary arrive in New York City, they track down Dr. Gwyn Davies, who isn't the scientist they expected. Astra, Spooner, and Gideon are still trying to stop the Legends from using the time machine, but they run into some more setbacks along the way. Meanwhile, in the pocket dimension Nate and Zari work together on the Hoover situation as well how to make their future together work.|
- Caity Lotz as Co-Captain Sara Lance
- Tala Ashe as Zari Tomaz
- Jes Macallan as Co-Captain Ava Sharpe
- Olivia Swann as Astra Logue
- Adam Tsekhman as Gary Green
- Shayan Sobhian as Behrad Tarazi
- Lisseth Chavez as Esperanza "Spooner" Cruz
- Amy Louise Pemberton as Gideon
- Nick Zano as Dr. Nate Heywood
Special appearance by
- Giacomo Baessato as J. Edgar Hoover
- Chris Britton as Thomas Edison and Thomas Edison
- Matthew Clarke as Erwin "Cannon Ball" Baker
- Charles Siegel as Sweet Old Man
- Eric Regimbald as Police Officer
- Eric Gustaffson as Shady Young Man
- Brian McCaig as Doctor
- The title of the episode is a play on the film It's a Mad, Mad, Mad, Mad World.
- Many of the Legends remark on Gwyn Davies looking familiar, a reference to Matt Ryan formerly portraying John Constantine on the show. Gary compares him to Mick (albeit with a beard), Sara likens him to a younger Stein, and Zari states that he looks like Ray.
- The room Gwyn Davies is committed to in the mental institution is number 237; the same as the hotel room from The Shining.
- In the official press release for the episode, Adam Tsekhman's name was misspelt as "Adam Tsekham".
- Eric Gustafsson is mistakenly credited as "Eric Gustaffson" in the end credits.
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