Kara is reunited with her cousin when Superman pays a visit to National City, and the two team up to stop an assassin who is targeting Lex Luthor's sister, Lena. Kara is faced with choosing her new position at CatCo, and the DEO takes custody of the stranger found in a crashed Kryptonian pod.
Cadmus unleashes its new project, the kryptonite-powered Metallo, on Supergirl and her cousin. Alex finds a new lead on the mole inside the DEO, while Kara struggles to deal with the changes in her life -- including her new boss, Snapper Carr.
National City and the DEO welcome the President of the United States for the signing of a bill opening citizenship to aliens -- and Supergirl soon finds herself defending her from an alien attacker. Meanwhile the new arrival wakes up and escapes the DEO, and Kara and James both struggle with their new roles at CatCo.
Supergirl and Alex find themselves in the middle of an alien fight club run by a powerful criminal who goes by the name Roulette, and are shocked to discover who her champion is. Meanwhile Kara has trouble trusting the Daxamite Mon-El.
Kara tries to tutor Mon-El in living on Earth in disguise by getting him a job at CatCo, as a group of criminals start a reign of terror in National City after they obtain powerful alien weaponry. James makes a decision about his future, and goes to Winn for help in making it happen.
An alien parasite is uncovered in the Arctic ice and infects a climate scientist, who feeds on Supergirl's powers -- pressing James to seize his own heroic destiny. Meanwhile Alex decides to share her secret with Kara, and Mon-El takes up a less than desirable new career.
Guardian becomes a citywide pariah after another vigilante begins murdering his criminal targets. CADMUS uses Mon-El to lure Supergirl to their stronghold, where she encounters the original Hank Henshaw. Meanwhile, Alex confronts Maggie about their relationship.
CADMUS unleashes a deadly virus that targets aliens and leaves Mon-El in quarantine, prompting Kara to try and learn what Lena really knows about her mother. Meanwhile Alex struggles to reveal her truth when her mother visits for Thanksgiving, and a pair of visitors arrive from a parallel dimension.
With help from Mon-El Kara investigates the disappearance of several of National City's residents, only to find a portal that leads to alien slave trafficking ring on another planet -- under the direction of Roulette. Winn has a crisis of courage, and Alex hurts her new relationship by lashing out impulsively.
Mon-El begins superhero training with Kara, but they must turn their attention elsewhere when mysterious forces break Livewire out of prison. J'onn must decide whether to help M'Gann when the White Martian comes under a devastating psychic attack, and James decides to share his alter-ego with Kara.
A White Martian from M'gann's past shows up to bring her to justice for her war crimes, putting J'onn and the rest of the team in harm's way when the DEO must go into lockdown -- not knowing which of them is the enemy in disguise.
During her trial Lillian Luther offers to make amends with her daughter, but suspicions are cast on Lena after Metallo escapes from prison with a new Kryptonite charge. As Kara tries to defend her friend, the Luthor matriarch puts her latest plan into motion.
Kara learns that she is the object of affection for Mr. Mxyzptlk, a magical imp from another dimension, who declares his love while wreaking havoc on National City -- and straining her relationship with Mon-El. Alex and Maggie clash over whether to celebrate their first Valentine's Day.
The Danvers family is overjoyed when Jeremiah is rescued from Cadmus, returning to work at the D.E.O. -- with a warning that the enemy is planning to use a powerful fusion bomb. But when Mon-El voices concerns about Jeremiah, the happily reunited family begins to fracture.
When Cadmus begins to abduct people from the alien registry Alex finds herself torn between her duty and her family, as Jeremiah helps Cadmus move forward with the next phase of their plot. Kara draws upon Supergirl to try and convince Snapper Carr to expose the new threat to the city's aliens.
Mon-El has some explaining to do when his parents arrive in Earth orbit in a Daxamite cruiser, revealing his secret to Kara. Winn finds himself in trouble with the law after Lyra talks him into breaking into the National City Museum.
When a high-priced alien bounty is place on Supergirl's head National City is inundated with bounty hunters from across the galaxy, and Mon-El suspects his parents. Meanwhile, Alex and Maggie's relationship is complicated when they run into Maggie's ex-girlfriend.
When Lena's ex-boyfriend comes to town to announce a revolutionary innovation in nanotechnology, Kara finds herself in the middle of a deadly investigation into a potential cover-up. While Lena considers reconnecting with her ex, Jack, Guardian receives some unwanted help on the streets.
Maggie and Kara work together to find Alex after she is kidnapped by a man looking for leverage to get his criminal father released from prison. Meanwhile, Rhea makes Lena a business proposal.
When an alien from a normally peaceful race attacks a public square, James Olsen may be the only one who can find her by getting through to her son. Lena moves forward with her project with Rhea, unaware of the bigger consequences.
When Rhea launches a full-scale Daxamite invasion of Earth, the President orders the D.E.O. to use a powerful weapon against the mothership -- while Supergirl joins forces with an enemy to try and rescue Lena and Mon-El. Cat Grant returns to National City.
After Rhea pits Kara against Superman himself, she takes the only option she can see: challenging the alien queen to single combat to save the planet. When the Luthers offer a deadly back-up plan, Supergirl faces the possibility that her future with Mon-El may be the price of saving her world.
