The CW's annual Arrowverse crossover has become a massive television event. What started as a pair of episodes that featured characters from Arrow and The Flash interacting has evolved into movie-length stories involving multiple Earths, heroes-turned-villains, and body-swapping comedy. The crux of the crossover is to celebrate the comic world that it emulates, highlighting both stakes and levity when appropriate. These yearly events are always packed to the brim with hidden nods to the comics, television, and films.
"Elseworlds," the 2018 crossover, brings together a lot of previously unseen realms, from Earth-90 to Gotham City. It's going to take a keen eye to catch everything, as there are plenty of Easter eggs to uncover in each episode.
After Oliver Queen and Barry Allen undergo a Freaky Friday-style body swap, there are reports of red skies and lightning storms over Central City. This is an Easter egg from both the comics and the shows. In the comics, red skies appear at the beginning of the Crisis on Infinite Earths storyline. In the first season of The Flash, Barry finds a newspaper from 2024 with the headline reading "Flash Missing Vanishes In Crisis." A smaller article on that same front page reads "Red Skies Vanish."
This - along with the Monitor's inclusion in the episode - provides ample evidence that "Elseworlds" is setting up the Arrowverse's own Crisis on Infinite Earths.
Another cell Ollie and Diggle walk past belongs to Guggenheim, M. This is a shout out to executive producer and "Elseworlds" writer Marc Guggenheim.
After vibing the past conversation between the Monitor and John Deegan, Oliver makes a superspeed drawing of what he saw and points out the meeting took place in Gotham City.
The art itself was done by legendary comic artist Jim Lee.
Oliver and Barry eventually head to Earth-38 to get Supergirl's help with their situation. After a bit of training, Cisco shows up to warn them about a robot destroying the city, and they head back to Earth. Supergirl and Superman decide to tag along, and when Cisco asks Clark who he is, he simply says, "A friend."
This is a callback to Christopher Reeves saying the same thing when he meets Lois Lane in Superman: The Movie.