Mon-El was probably the most prominent addition to the (then) regular cast of Supergirl in season 2 outside of the now-departed Maggie Sawyer. But while the former's recently-concluded love story with Alex Danvers was among the series' most positively received non-superheroic storylines, fans remained split on the concurrent romance between Kara Zor-El and the alien prince trying to reform his bad-boy ways following the destruction of his Krypton-adjacent homeworld. Many fans were glad to see Kara in a relationship that defied the typical "super person/civilian" dynamic of the genre and found the pair to have good chemistry, while others objected to the scuttling of Season 1's teasing at a pairing with James Olsen or never warmed to the particular character.
Either way, the relationship storyline came to an end at the conclusion of season 2 when Mon-El's evil mother rolled in as the main villain of the final stretch and the only way to be rid of her was to render Earth's atmosphere poisonous to his species - forcing him to depart the planet in the same shuttle he'd arrived in. Thus far, season 3 has largely dealt with Kara working through the fallout of how the relationship was made to end; withdrawing from human relationships and embracing her "alien-ness" on the premise that she may be effectively pre-destined to not be fully of Earth (as in the original comics, Kara differs from her cousin Clark Kent in that she arrived on Earth as a teenager having lived a "normal" life on Krypton.)
But as of Monday night's "Wake Up," the status-quo has been shaken up once again: The "ancient" sunken alien space ship full of mysterious sleep-pods, first teased as a stinger several episodes episodes ago, finally got explored by the DEO and was discovered to be protected by a returned (and now, somehow, breathing fine) Mon-El... and then things got much, much more complicated. Supposedly, the ship and its passengers are from Saturn - which is notable since (even in Supergirl's universe) Saturn isn't inhabited.
That the ship appeared to have been stuck on the bottom of the sea for around 12,000 years might have explained that; but Mon-El's eventual coming clean about how he's back and why he's acting "distant" end up going in an entirely different direction (though, admittedly, one that fans well-versed in DC Comics lore had probably guessed sometime around "Saturn"). The ship is actually a time-unstuck vessel from the future, specifically the 31st Century - where Mon-El has been living (on Earth, no less, where the L-Corp of the 3100's had managed to switch of the anti-Daxamite atmosphere) ever since his shuttle got pulled into a wormhole. Stranger still, while he's only been gone for seven months in "our" timeline, for him it's been seven entire years. Oh, and the only other (currently) awake passenger on the shuttle happens to be Imra Ardeen - his wife.
For DC fans, that name is big news and confirms what most such viewers had been guessing at all along (unless they'd just read the confirmation from the producers months ago): Imra Ardeen is better known as Saturn Girl, and her arrival marks the start of the CW "Arrowverse" venturing into the complicated mythology of The Legion of Superheroes; traditionally a team of alien teenage superheroes from the galactically-intermingled 31st Century originally introduced in Silver Age Superboy stories and still in (somewhat convoluted) existence to this day.
Exactly how The Legion will be utilized on Supergirl remains to be seen (a Legion Flight Ring of the type seen in the comics was one of the spotlighted "easter eggs" among Superman's personal museum in the Fortress of Solitude back in Season 1 - but whether that actually ends up being called back to is anyone's guess at this point); though the fact that Imra seemed to be aware of and impressed to meet Supergirl "herself" suggests that it might follow the same track as the original Superboy stories: There, The Legion was ultimately revealed to come from a future where superheroics had passed out of fashion and revived the tradition as a rebellious youth-culture trend by following Superboy's historical example.
More immediately, though, the series (and fans) are likely to be more concerned with the more personal ramifications of all these revelations for the main characters. While the time-differential conceit is a handy way of allowing the storyline to play out with Mon-El coming off at least somewhat sympathetic; the idea that Kara has effectively been unable to function outside of her Supergirl persona (read: at work) without him while meanwhile he's moved on and built an entire new romantic life for himself is about as crushing a reveal as could be contrived with or without the science fiction angles that make it possible here.
The reveal was especially jarring (conceptually, at least - in terms of plotting you can kind of see it coming well in advance even if you've never heard of The Legion or Saturn Girl) because it comes as part of an episode that's otherwise busy with series housekeeping ahead of next week's big Crisis Crossover with the rest of The Arrowverse: J'onn J'onzz and his newly-rescued father decided to move into an apartment together after realizing that J'onn lacks a social life outside of work (and apparently has for a few hundred years now) and "Reign" going through her own twisted-up version of the Superman origin; wherein she ran away from home before her adopted mother could tell her about her real origins and now gets her own "evil" version of The Fortress of Solitude in the desert rather than the North Pole.
Exactly where Supergirl plans to take this story should be interesting to see, even as many fans are currently quite unhappy to have Mon-El be back in a "betrayal" storyline (or, alternately, to have him back at all.) The series has often been credited positively for not following "typical" storylines for female-led series like romantic rivalries over men, which would seem the otherwise obvious place to go with Mon-El showing up married to another (presumed) superhuman woman. Whether fans will be "there for it" remains to be seen, but if nothing else it's an unexpected turn for a series that's been built on them.
Supergirl airs Mondays @8pm on The CW.