During the episode, Mon-El makes a passing reference to the Legion of Super-Heroes battling General Zod in the future -- which surprises Winn. Why?
"Superman killed Zod," Winn objected, before connecting that it was likely the Kryptonian general was somehow revived.
Killing Zod in Man of Steel was a hugely controversial move, and with this throwaway line of dialogue, the show has established that Tyler Hoechlin's smiling, charming Superman also made that decision at some point in his life.
Of course, that does not by any means indicate that Zod's death happened in the way it did in Man of Steel, much less (as some people inevitably asked on Twitter) that the two share a continuity.
In fact, when fans first started to complain about the death of General Zod in Man of Steel, one of the defenses that writer David S. Goyer came up with was the idea that John Byrne had done something fairly similar during his status quo-shattering run on Superman and Action Comics in the late '80s.
"Superman has killed Zod before," the writer told me at Comic Con in 2013. "We didn't invent that."
That year, the 75th anniversary of Superman's first appearance took center stage, and fans were eager to ask about the then-new movie and its implications for the cinematic Superman.
"Our advantage was that we had another thirty comics we published that year," former Superman group editor Mike Carlin said at the time, noting that they faced controversy when they did it, as well.
Still, he maintained, fans could much more quickly and easily see that the events of the story had taken a toll on Superman and what that meant for the character and the comic going forward.
In Superman #22 by John Byrne, the character was faced with Zod and two of his cohorts, who had extinguished all life in a parallel universe that included a version of Earth. Superman, who had been called in by that world's Supergirl to help, ultimately executed the Kryptonian criminals using Kryptonite (which did not affect him because he was from a different universe and Kryptonite was slightly different -- something that would be used again in Infinite Crisis and other stories dealing with Superboy Prime). The Kryptonians had been previously rendered powerless by gold Kryptonite, and while they threatened -- as Zod did in the movie -- that they would never stop, that they would find a way to travel to Superman's world and kill everyone there as well -- they gave up their bluster and pleaded for mercy when Superman produced the Kryptonite. As you can see in the panel at top, Superman was unmoved, putting them to death for their crimes against humanity.
Legendary Superman writer/artist Dan Jurgens actually said he believed the movie had done it better than Byrne did, because "If you wanted to have Superman kill the Kryptonians, I think it had to be a situation where innocent life was in immediate peril and the only way to stop them from taking innocent life was to kill them."
"At that point, Superman makes the same decision, but he's much more Superman as part of that," Jurgens added. "And the funny thing is, everybody gets twisted in knots over of that scene in the movie -- yet that's what Superman did. When Superman kills Zod in the movie, it's because there are human beings there who are in immediate danger. The problem with the comic book was, I always thought, not that Superman did it as it was the way he did it, because he was judge, jury and executioner right there. And it was a police officer walking right up to an individual who had dropped his gun, dropped his knife, said 'I surrender,' waved the white flag...and still [blowing] his head off."
The decision to kill Zod in the comics ultimately touched off a major psychological issue for Superman, which ended in his decision to briefly exile himself to space. During that time, he battled Draaga, an alien champion, in a gladiatorial arena (not too unlike what ended up happening in the Planet Hulk story that would eventually become Thor: Ragnarok's biggest subplot).
Draaga, of course, appeared as an antagonist in a season 2 episode of Supergirl, indicating that perhaps it is more likely the comics' version, and not the movies, that happened in this series' continuity.
We will not get into the whole hornet's nest of whether the Christopher Reeve Superman also killed Zod in Superman II...!
Supergirl airs on Monday nights at 8 p.m. ET/PT on The CW.