The episode opens with a massive Superman and Supergirl brawl. Rhea used Silver Kryptonite on Superman to make him think that Supergirl is Zod. The two battle back and forth, destroying parts of National City in the process. Supergirl pleads for Superman to fight it, but Superman keeps thinking Zod is taunting him and threatening to kill everyone she loves. Supergirl scores a major uppercut and knocks Superman unconscious and into the pool of water they were fighting in.
After the fight, Kara dreams that she's cuddling in bed with Mon-El. She explains to him the significance of her necklace (her mother gave it to her right before she left Krypton) before Mon-El says that she's not actually in bed, she's in the Fortress of Solitude. Supergirl wakes up and finds out that she flew Alex and Superman to the Fortress before she passed out. Afterwards, Superman wakes up, now cured of his Silver K poisoning.
Back in National City, Lena and Lillian have an argument over Lena's involvement with the Daxamite invasion. Lillian says that her cause was righteous, but that maybe she picked the wrong Luthor. She gives Lena a box from Lex Luthor's vault, which Lena can use to the save world.
Superman explains what happened to him and says that Supergirl beat him at full strength. Superman looks through his archives from Krypton and discovers something that could be used to stop the Daxamites.
Superman and Supergirl fly to the DEO and meet up with Winn and Mon-El. Supergirl reveals her grand plan: she challenges Rhea to a sacred duel - a one on one fight to end things once and for all. Rhea accepts the fight. Mon-El is upset, but Superman points out that she just beat him in a one on one fight, so she really is the champion of Earth.
J'onn is still unconscious, but M'gann telepathically asks him to wake up as the Great War is coming and his friends will need him. J'onn actually wakes up and asks Alex what the heck happen.
J'onn walks into the DEO's central command as the local media dubs Rhea vs. Supergirl as the Intergalactic Fight of the Century. J'onn says they need to contain the media, as he doesn't want bystanders hurt during Kara's fight.
Kara and Clark Kent show up at CatCo and ask Cat for a favor. "Supergirl" gave CatCo an exclusive interview, but she wants Cat to tone down the media's sensationalism to keep people from getting hurt. Cat agrees and tells Kara to let Supergirl know that everyone is rooting for her. Afterwards, Superman gets an alert saying that Lillian wants to meet.
When they show up to L-Corp, Lena explains their plan. Lex created a device that would put a trace amount of kryptonite into the air to force Superman to leave the planet. However, Lena can modify the device to put out lead instead. It would be harmless to humans, but would make the atmosphere toxic to all Daxamites....including Mon-El. Supergirl tells Lena to start working on the device, just in case.
Back at the DEO, Supergirl explains what the device does and Mon-El says to use it if necessary. J'onn sends Winn to help Lena, while Superman agrees to spar with Supergirl until her fight.
During their sparring session, Superman teaches Supergirl a move he learned on Warworld. Supergirl says that she's so close to having everything she wanted, but that she could lose it all if Rhea wins. Superman says that the people they love are their secret superpower because it makes them fight that much harder.
Supergirl and Mon-El head to a rooftop as Rhea and her general teleport in. Rhea and Supergirl agrees to terms (The Daxam army will leave if Supergirl wins, Earth will surrender if the Daxamites win) and then start the duel. The fight starts with some heavy blows, but Supergirl quickly gets the upper hand. When Rhea falls down, the Daxamite Army starts blasting National City. Mon-El flies off to help National City, leaving Supergirl alone with Rhea.
Superman and J'onn fly off to help National City, leaving Alex in charge of the DEO. Meanwhile, Winn and Lena finish the device, but when Lillian tries to turn it on, Lena says that she gave the remote to Supergirl.
Supergirl also faces a complication - Rhea's blood is laced with kryptonite from the destruction of Daxam. As Superman, Mon-El, and J'onn save the city, Rhea's fight with Supergirl spills through a construction site. Superman takes down Rhea's general in CatCo, while M'gann arrives with an army of (friendly) White Martians to even the odds.
Supergirl gets the upper hand on Rhea, but Rhea orders her army to target every hospital and school in the city. As Mon-El arrives to help Supergirl, he gives her a knowing glance. Supergirl pushes the button to Lena's device, causing it to go off. As Daxamite soldiers begin dying of lead poisoning, their fleet quickly moves off planet. Rhea begs Mon-El for help, but Mon-El refuses. Rhea petrifies and dissolves into dirt. Mon-El starts coughing as the lead starts to affect him, so Supergirl asks Alex to pull out her rocket.
Supergirl gives Mon-El her necklace before he leaves and forces him to get in the rocket. Before he flies off, he says he'll be a better person because of her and Kara says she loves him. Mon-El manages to flies away as Supergirl watches and cries.
That night, National City and the DEO celebrates for surviving the DEO invasion. Superman comforts Supergirl and says that he's humbled by what she did. Superman reiterates that Supergirl is stronger than he is and then leaves to go to Metropolis.
M'gann says that she's leading the resistance on Mars, but that she left for Earth because she knew J'onn needed her. They kiss and M'gann says that she'll stay on Earth for a while.
Alex comes out and comforts Kara by saying that she's proud of her. Alex offers to stay with her, but Supergirl just wants her to be happy and tells Alex to never let Maggie go. She then flies away as Maggie approaches. Alex immediately asks Maggie to marry her. Maggie doesn't say yes, but she does give Alex a big smile.
The next day, Kara comes into CatCo. Cat sees her frowning and asks what's wrong. Kara explains that her relationship just ended and that she thinks that she's destined to be alone. Cat says that what women strong is that they have the guts to be vulnerable. Cat also points out all Kara's accomplishments over the last years and says that she's on a hero's journey. Kara thanks Cat and then runs off when she hears sirens. As Kara leaves, Cat reveals to herself that she knows that Kara is Supergirl.0comments
As Supergirl flies off to save the day, Mon-El is shown flying into an unknown wormhole.
The episode ends with a flashback - right as Kal and Supergirl's rockets leave Krypton, a group of cloaked Kryptonians put another baby in a rocket. They refer to the baby as an "it" and feed it blood and says that "it" will reign over Earth. The rocket leaves Krypton...and right towards next season.