- "And what are you to my daughter?"
"I'm a friend."
- "I've heard that before. Years ago, Superman came to my son, Lex, and promised him the world. A new dawn for mankind. Lex believed him, poor thing. And then he watched as your kinsman infected this country with his propaganda, as his power became unchecked. And when Lex tried to take that power back, Superman convinced the world Lex was evil. My darling boy. My genius son. An actual superman, behind bars for life. Because you and your kind will stop at nothing to poison Earth."
- —Lillian Luthor and Kara Danvers
|While Guardian tries to clear his name after being accused of a murder committed by another vigilante, Supergirl heads out on a solo mission to rescue Mon-El who has been captured by Cadmus. While fighting to free Mon-El, Supergirl comes face to face with Cyborg Superman.|
Twenty-four hours earlier: Alex, Kara, Winn, and James hang out at Al's Dive Bar. A newsflash of Guardian appears on the bar's TV. Kara remarks that she disagrees with Guardian's efforts in the city, but James defends Guardian's actions. A brief flashback shows Guardian taking out 3 robbers. Kara doesn't like that Guardian wears a mask. However, Winn compliments the suit. Alex theorizes that Guardian has a partner. Another brief flashback shows Winn directing James as Guardian from a van. Alex then introduces the gang to Maggie. Maggie takes Alex aside and tries to clarify their relationship since they spoke last. She asks Alex if they can still be friends and Alex says yes. Afterwards, Alex asks Kara where Mon-El is. Kara assumes he is at some girl's apartment.
Meanwhile, Mon-El is in a cage at Cadmus. He tries talking to the guard and gets the guard to aim a gun at him through the bars. Mon-El then disarms the guard and steals his keycard, using it to attempt escaping. Lillian Luthor appears at the doorway and threatens "J'onn" so Mon-El stands down.
M'gann checks in on J'onn after his blood transfusion from her. After M'gann gives him some traditional Martian sick remedy, he hallucinates seeing his family.
Guardian takes out a liquor store robber and leaves him hanging. After Guardian leaves, another vigilante shoots the robber dead.
The next day, James walks into CatCo to the news that Guardian has murdered someone. Snapper shows him surveillance footage of last night that seemingly implicates Guardian. Winn shows up and tells James to call off his vigilante activities. James refuses and vows to catch the gunman that night at the wharf.
At the D.E.O., Kara catches J'onn meditating. J'onn tells her of his hallucinations. Kara tells him that she used to see visions of her family as a child and how Eliza helped her cope. She tells him that seeing M'gann may be causing him to reminisce.
Guardian takes out some drug dealers at the wharf. The killer vigilante shows up and they both fight. However, the other vigilante gets away as the NCPD arrive. Guardian attempts to escape from them. While in bound to catch Guardian, Supergirl is telepathically contacted by Lillian Luthor telling her that Cadmus has Mon-El and to show up alone. Supergirl runs into Hank Henshaw, at first thinking it was J'onn. Henshaw says that Cadmus has saved him and that they now carry out his original goal of the D.E.O. to eradicate aliens. Supergirl punches Henshaw, but had minimal effect. Henshaw then proceeds to throw her into a wall. Kara blasts him with heat vision revealing his cyborg skeleton underneath. Henshaw repeatedly punches Supergirl and knocks her out.
Alex asks Winn if he has heard from Kara. Winn assures her she must be at CatCo. Winn also tells her that Kara needs to lay off of Guardian. Alex says he's a murderer but Winn insists that he is not a murderer. Alex forces him to reveal that Guardian is James.
Kara wakes up in a cage next to Mon-El. Kara tries to escape but Mon-El tells her the cage is made of Nth metal.
Alex meets up with Maggie to tell her to drop the Guardian case. Maggie asks why since they're friends but Alex tells her they aren't friends. Alex confesses to Maggie that she was the one who convinced her that her feelings were real and to come out to Kara but that all she feels now is pain.
Alex fails to contact Kara. J'onn advises to wait before panicking. J'onn then hallucinates seeing a White Martian and points a gun at it, but Alex talks him down after he realizes it's just another D.E.O. agent. J'onn tells Alex that something is wrong with him and to run tests.
Lillian walks in a reveals to Supergirl that Lena Luthor is her daughter. Supergirl asks her if Lena knows what she has been up to. Lillian tells her she blames Superman for turning the world against Lex and blames Superman and all aliens for poisoning Earth. Lillian then moves on to her purpose for kidnapping Supergirl: to get her to solar flare. She shoots Mon-El with a lead bullet and threatens to kill him if Supergirl doesn't cooperate. Supergirl gives in and agrees to solar flare into a specialized helmet. Lillian orders guards to take her away for experimentation. Lillian then takes a sample of her blood.
Winn shows up to CatCo with a plan to catch the killer vigilante. Winn used his investigative skills to identify the other vigilante as Phillip Karnowsky and that he was targeting criminals who had gotten off due to judicial technicalities. Winn tells James he knows the next target.
J'onn realizes that these are more than just simple hallucinations. Alex tests J'onn's blood and finds anomalies. J'onn knows what happened and confronts M'gann outside Al's Dive Bar. M'gann admits that she is a White Martian but pleads to J'onn that she is not like the others. She tried to help Green Martians to safety but barely escaped with her own life. J'onn demands M'gann show her true form; they both morph into their Martian forms.
Kara is put back into the cage. Kara tells Mon-El she's scared and that if she doesn't make it, to tell Alex to live her own life on her own terms. Mon-El says he deserves to die like the rest of his people but Kara says the prince sacrificed his life for his. Mon-El is about to admit something to Kara but Jeremiah arrives to take Kara away. Jeremiah takes out the lead bullet from Mon-El's leg and ushers them to a hallway. Jeremiah tells them he has to stay behind to slow them down. Kara tells him that Alex would never forgive her and hugs Jeremiah before he goes.
J'onn and M'gann fight in the sky. J'onn, filled with grief and vengeance for his people, attacks M'gann. He holds her down by the throat but stops himself from killing her.
Guardian finds Karnowsky's victim tied up. Karnowsky arrives and opens fire on Guardian. Guardian uses the grappling hook to avoid the bullets but Karnowsky downs James with grenades. Karnowsky then moves to kill his target but James knocks him back. They get into a fist fight. Karnowsky pulls a gun on Guardian. Guardian pleads to Karnowsky that this isn't what his late wife would want, but Karnowsky calls him a fool and shoots him just as the NCPD show up. The bullets ricochet off the shield and Guardian uses his grapple to hurl Karnowsky to the feet of Maggie and Alex. Maggie lets Guardian go.
Alex races back to the D.E.O. to see Kara. Kara tells her about Jeremiah's survival. Alex leads a strike team to the Cadmus hideout to find it has been cleared out.
J'onn checks on M'gann's prison cell. J'onn refuses to listen to what she has to say but M'gann tells him that the blood transfusion is slowly turning him into a White Martian.
Alex, Winn, and James show up to Kara's apartment with pizza and potstickers. They see the news exonerating Guardian of the murders so Kara fesses up to being wrong about Guardian. Kara tells Mon-El that he was brave and that there is a hero within him. Mon-El asks James and Winn if Kara is mated to anyone. They tell him no and that Humans mostly chose their own mates. Mon-El smiles while looking at Kara. Maggie shows up to the apartment and talks with Alex outside. Maggie says that she cares about Alex and that one day she hopes they can be friend. Alex invites her to play pool tomorrow night. Kara promises Alex that they will to find Jeremiah.
- Kara mentions how her cousin "worked with a vigilante once, tons of gadgets, lots of demons". This is a reference to Batman.
- Clark's vigilante friend is confirmed to be Bruce Wayne in .
- It is revealed that the Green Martians were not all wiped out by the White Martians; some were transformed into White Martians by being injected with White Martian blood.
- Mon-El mentions Thanagar. In the comics, Thanagar is the home world of the Silver Age Hawkman and Hawkgirl.
- In addition, Thanagar also plays an important part in the plot of .
- Mon-El also states that the Nth metal in their cells originated from Thanagar, a fact known in the comics.
- Mon-El mentions that Daxamites are often paired off at birth, and one day find themselves mated to someone. This might be an allusion to Lois & Clark: The New Adventures of Superman, where a major story arc is centered around Clark finding himself in that exact predicament.
- Phillip Karnowsky is referred to as an Executive Officer in the Navy according to the file Winn Schott gives James. This is incorrect as an Executive Officer is usually a second-in-command. Phillip's shoulder patch shows two red chevrons, indicating his rank as a Petty Officer Second Class.
- Jeremiah claims to have been held captive by Cadmus for 15 years. However, since he began working for the D.E.O. shortly after Kara arrived on Earth in 2003 and disappeared a year later while Kara hid her powers for 12 years and has been using them for one, Jeremiah should've been captive for 12 years.
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