In the third season finale, M'yrnn gives his life to stop the terraforming. But the Daughters of Juru will not be so easily defeated, nor will Reign. Meanwhile, Winn and Alex receive offers they can't refuse.
Tropes:
- 11th-Hour Ranger: Samantha's inward spiritual journey unlocks her Kryptonian powers, allowing her to join in the battle against Reign and the witches. Her powers are only temporary, however.
- Almost Dead Guy: Coville lives just long enough after getting blasted last episode to activate the emergency beacon on J'onn's ship.
- Amicable Exes:
- Mon-El and Imra. Imra even remarks the distance made her realize that their marriage was never going to work since it was never based on love.
- He and Kara in the end too, as he is forced to leave her again, but she understands.
- And This Is for...: Sam gives Reign a beatdown for all the things she did to Ruby, her mother, and finally herself.
- The Atoner: Coville sends a beacon to alert Supergirl to the Witches' location.
- Back for the Finale:
- Imra and Brainy. (Though Brainy appears destined to stay for a while.)
- Although he's just in the background (as in the literal opposite end of the planet to National City), Superman is finally acknowledged as having some involvement in the proceedings.
- Batman Can Breathe in Space: Averted - Kara needs the Legion Ring to do so.
- Bittersweet Ending: M'yrnn gives his life to stop the terraforming. Mon-El, Alura, and Sam die in the final battle, but Kara uses Mon-El's Legion ring to back through time and stop it. Reign is stopped by Kara and Sam, but as a consequence there's another Kara in Siberia. However, Mon-El still has to return to the future to stop Brainiac, and Winn goes with him. Mon-El has to sacrifice his relationship with Kara, while at the same time terminating his marriage to Imra. But Selena and her cult will stand trial on Argo, and Sam is finally free from Reign and is human again.
- Brought Down to Normal: Destroying Reign causes Sam to become completely human.
- The Bus Came Back: Imra and Brainy come back to Supergirl's time.
- But Now I Must Go:
- Mon-El has to return to the future to stop Brainiac. And Winn decides to go with him.
- Alura returns to Argo with Selena and company in tow to stand trial.
- J'onn steps down as Director to travel the Earth and live amongst humans.
- Cosmic Retcon: Kara takes a page from Barry's book and uses Mon-El's Legion ring to go back in time to stop Mon-El, Alura, and Sam from being killed.
- Death Is Cheap: Sam's new powers means she's still inexperienced, so of course she gets killed in the final battle (though this is undone by Supergirl using the Legion ring).
- Did Not Get the Girl: Or girls in Mon-El's case. After all the back-and-forth, he ends up with neither Imra nor Kara.
- Didn't See That Coming:
- Turns out that stopping the Blight saved billions in the future... including the original, evil Brainiac.
- Selena and her followers are completely unaware of the existence of Kryptonite, leaving two of them blindsided when their attackers use Kryptonite handcuffs on them.
- Dragged Off to Hell: When Reign is forced to drink the weakening water of the fountain, the dark spirits show up and drag her into the water.
- Do Not Go Gentle: Reign's death in the original timeline. She is thrown into the cauldron, which slowly destroys her body. Screaming in agony, she fires off her heat vision in all directions, killing Alura, Mon-El and Sam before she finally perishes.
- Face Death with Dignity: M'yrnn is calm and collected before his final moments, comforting his son.
- Heroic Blue Screen of Death: Supergirl has one after killing Reign, which (temporarily) results in the deaths of Alura, Mon-El and Sam.
- Heroic Sacrifice:
- M'yrnn gives his life to stop the terraforming of Earth.
- Mon-El gives his life to stop Reign from killing J'onn, though Kara undoes it with Time Travel.
- Hero Killer: Before its undone by Kara, Reign kills Sam, Alura and Mon-El.
- Hero of Another Story: Superman is busy doing damage control in Madagascar.
- History Repeats: A Badass Normal Vigilante Man who is secretly a well-off and prominent individual outs himself to the public, much like what happened in the Season Finale of another Arrowverse show.
- I Choose to Stay: Kara decides to remain on Earth, since her (proper) paranoia in "Not Kansas" made her feel like a Stranger in a Familiar Land and sees Earth as her home now.
- Internal Reveal: James publicly confesses that he's Guardian.
- In the Back: How Sam tries to kill Reign. Unfortunately, it doesn't work.
- Jumping Off the Slippery Slope: Lena lied about giving all of the Harun-El to Alura. She still has enough left to experiment with on her own; as a scientist, the temptation was too much.
- Killed Off for Real: Although most of the deaths in the episode are undone, M'yrnn is gone for good.
- Last of His Kind: With his father's death, J'onn truly becomes the last Green Martian (barring future plot twists).
- Mirror Match: The showdown between Sam and Reign.
- Mistreatment-Induced Betrayal: The witches deciding to dispose of Coville when he's of no more use to them leads to him alerting Supergirl and the DEO to their location.
- Mythology Gag:
- J'onn leaving the DEO to live among humans is just like his final arc in Justice League Unlimited.
- Supergirl turning back time to undo the death of a loved one (more than one, in this case) reflects the finale of Superman: The Movie, as well as the "Flashpoint" arc of The Flash (2014).
- The Supergirl Doppelgänger waking up in Siberia is a reference to Kara's introduction in Last Daughter of Krypton, where Kara's pod crash-landed in Siberia.
- Nice Job Breaking It, Hero: The Legion meant to save lives by stopping Pestilence from evolving into Blight. They succeeded, but now the villainous Brainiac is alive again in the future and is bent on destroying all other AI's in the universe. ...Oops.
- Non-Action Big Bad: Despite them previously being a major threat to normal humans, it turns out none of the three Kryptonian witches fare well in combat when going up against other Kryptonians like Kara or Alura, hence why they typically prefer to have Reign do the dirty work for them. Two go down almost instantly thanks to some Kryptonite handcuffs, while Alura pretty handily beats Selena. They can't even use the excuse of not being familiar with having superpowers considering Alura adapts very quickly and has had even less time than they have.
- Not Quite Dead: Coville, having survived the Heat Vision blast he received from Selena.
- Platonic Life-Partners: When Winn is saying his goodbyes, it's clear that he and Alex have grown this close, even though there was never any romance between them (she being a lesbian).
- Poorly Disguised Pilot: Although the entire Legion arc serves as a try-out for a potential future spin-off, this episode is even moreso as it ends with Winn and Mon-El returning to the future to defend against Brainiac. While such a storyline might cross back to a future Supergirl arc, it also puts a number of pieces into place for a spin-off, should one be commissioned.
- Put on a Bus: Both Chris Wood (Mon-El) and Jeremy Jordan (Winn) depart as regulars here, as does Odette Annable (Sam/Reign).
- Rank Up: J'onn promotes Alex to Director when he plans to go Walking the Earth.
- Reset Button: First, there's Supergirl going back in time to undo the deaths of Sam, Mon-El and Alura (not to mention her own first intentional kill). Somehow, she also manages to undo the deaths of Brainy and Imra caused earlier when she and Reign smash into their spaceship.
- Resignations Not Accepted: Alex tries to leave the DEO to start a family, but J'onn refuses... because he's promoting her to be the new Director. That way she can have her career and still be a mother.
- Retired Badass: J'onn retires after the Earth is saved.
- Season Finale: The third one.
- Set Right What Once Went Wrong: Kara uses Mon-El's ring to negate the deaths of Mon-El, Samantha and Alura.
- Shout-Out:
- Kara makes a Star Wars reference and is quite aghast when Brainy doesn't get it.
- In the dream world, the wraiths drag Reign down the "death spring" in the same manner as the wraiths do to evil people in Ghost (1990).
- Skewed Priorities: When Brainy doesn't get the Star Wars reference, Kara takes a moment to ask if they ever got around to showing him those movies. This is while they were working to contain the numerous natural disasters caused by Reign's attempted terraforming of Earth.
- Spirit Advisor: Patricia shows up in the valley to keep Sam from drinking from the wrong side of the fountain.
- Spoiler Title: Even if one doesn't watch the episode, its Bittersweet Ending is easy to guess just by glancing at the title.
- Star-Crossed Lovers: Once again, Mon-El and Kara are forced to part ways.
- Status Quo Is God: In order to keep Supergirl the main super-powered being on the show, Sam reverts to human after Reign's death, Alura returns to Argo City, and Mon-El returns to the future.
- The Stinger: The episode ends with Lena revealing she still has a piece of the Harun-El to experiment on, and a doppelgänger of Kara inexplicably shows up in Siberia. Also counts as a Sequel Hook in this case.
- Superman Stays Out of Gotham: As per usual for the story arc, however at least this time he's at least name-checked as taking care of damage being caused at the opposite end of the planet. That said, there's still no indication that he ever got a chance to see his aunt before she returned to Argo City.
- A Taste of Power: Samantha briefly gets Kryptonian powers to aid in the fight against the Worldkiller cult, but by the episode's end, her DNA has reverted to being 100% human.
- Trapped in the Past: Brainy is forced to stay in the past because the original Brainiac released a virus that is killing all AIs in the future.
- Traitor Shot: Not as obvious as is normal for this trope, but when Alura says she's so glad that there won't be any dangerous Harun-El left on Earth, the camera immediately looks at Lena, and from the look on her face, it's clear that she's kept some and isn't telling anyone about it...
- Villain of Another Story: Following Pestilence's death in the present and the resulting non-existence of Blight in the future, Brainiac becomes the new major threat the Legion must face there.
- Walking the Earth: J'onn decides to do this after the day is saved.
- Wham Episode: Mon-El and Winn leave for the future and James confesses to being Guardian, while J'onn steps down as Director of the DEO and gives Alex his job, while he decides to wander the Earth.
- Wham Shot: The episode and season ends with a doppelgänger of Kara showing up in Siberia, implicitly a result of her contact with the Harun-El while attempting to banish Reign.
- What Happened to the Mouse?:
- Coville is last seen unconscious in the pilot seat of J'onn's ship, presumably having succumbed to his wounds, but the episode never follows up on it.
- Imra is never seen again after her spaceship explodes, though the fact Brainy is seen later implies she survived (assuming the crash wasn't undone by Kara reversing time).
- While listing all the reasons for why he can't just go to the future, Winn doesn't even mention his alien girlfriend Lyra, who hasn't been seen all season, or his mother, with whom he has reconnected after all these years.
- You Are in Command Now: J'onn surrenders control of the DEO to Alex, as he wants some time off.
