In National City, Kara, aka Supergirl, joins her cousin, Clark Kent, Superman, to stop new threats by outsiders. When a pod crashes to Earth it leaves J’onn and Supergirl stunned to see who is in it. It turns out it’s Mon-El, a survivor and prince of a royal family from the planet Daxam who seeks to become a superhero on Earth due to the fact that he has has similar powers to Superman and Supergirl.
In, “The Last Children of Krypton,” Cadmus assaults National City with kryptonite and winds up doing some serious damage to Supergirl. Cadmus has made it a priority to eliminate all alien life on Earth, including Supergirl and Superman, by any means necessary. Superman is convinced J’onn is involved based on the grounds that the Kryptonite was stolen from the D.E.O.
In “Welcome to Earth,” an assault is made on the President and Supergirl and the D.E.O. are appointed to secure the President and capture the ones responsible. Alex collaborates with Detective Maggie Sawyer to examine the case while Kara looks for Mon-El, who she fears might be the outsider behind the strike.
Later, Kara and Alex find out about an alien fight club ran by Roulette, an unregistered outsider and the ringmaster who orders super-powered villains to battle to the death. While attempting to stop the battling, Supergirl and Martian Manhunter are forced to fight a shocking adversary. All the while, Hank is excited to have discovered another Martian and tries to become more acquainted with M’gann.
Later, Cadmus sends a video to the D.E.O., the group acknowledges Cadmus is the one supplying the new group of outsiders for a mystery mission. Then, Mon-El takes on a new identity as he makes his presence felt in CatCo after Kara gets him the job. Meanwhile, James settles on a critical choice and Lena welcomes Kara to go to one of her pledge drives.
A new character, Guardian is introduced to help after a rogue outsider channels Supergirl of her energy. He is actually Jimmy Harper working the streets as a cop by day, then he becomes the Guardian in his off hours, trained and ready to fight.
While Guardian tries to demonstrate his innocence after being blamed for a murder done by another vigilante, Supergirl takes off on a mission to save Mon-El who has been caught by Cadmus. While battling to free Mon-El, Supergirl encounters Cyborg Superman.
In “Medusa,” Cadmus releases an infection that immediately slaughters any outsider in the region so Kara enrolls a startling partner, named Lena Luthor. The epic hero action commences at the same time Barry Allen, the Flash and Cisco Ramon from Team Flash go on a mission with Kara to find an outsider that attack their Earth.
Kara and Mon-El face Roulette once again after being sent through a portal to a planet called Slaver’s Moon, where people are sold as slaves, while trying to find a missing woman named Izzy. To make matters worse, Slaver’s Moon has a red sun, which strips Kara and Mon-El of their forces and they’re now stuck on the planet with no way home. In the meantime, back on Earth, Alex blames herself for Kara’s vanishing.
Later, Armek, a White Martian, arrives in National City on a mission to take M’gann back to Mars to confront her discipline as a swindler. J’onn and Supergirl decide the most ideal approach to protect M’gann is to convey her to the D.E.O.. Nonetheless, when it is uncovered that Armek shape-moved into M’gann and is presently free in the building, the group acknowledge he could be veiled as any of them.
At the point when Jeremiah Danvers is safe from Cadmus, Alex and Kara are excited to have their dad back. They have a family supper to celebrate, only things go astray when Mon-El begins to ask Jeremiah about his sudden return.
Then Kara examines a progression of outsider kidnappings, the sisters acknowledge they should break the rules to plan another Cadmus plot. With an end goal to get Snapper Carr to run a story that would help her stop Cadmus, Kara consents to set up a meeting amongst Snapper and Supergirl.
Later in “Star Crossed,” Kara having learned Mon-El’s true identity isn’t the main focus. Rather, the episode shines a light on Winn and Lyra’s budding relationship. After helping Lyra break into a National City museum for what Winn thinks is simple fun, he’s called into the police station by Maggie. It’s quickly revealed Lyra used Winn to steal Van Gogh’s “Starry Night” and set him up to take the fall.
Jack Spheer who is Lena’s ex, comes to National City to disclose his enormous leap forward in nano-innovation, which could possibly stop all diseases. Lena requests that Kara go to Jack’s meeting with her for help. At the point when Kara sees Snapper she’s reverts back to being a reporter to find out more. Only for it to lead to a bigger threat in which everyone is in danger.
At the point when National City and Supergirl face a series of attacks by that only Guardian can stop, Supergirl and the D.E.O. fight a Phorian, an alien with supernatural forces. They entrust James, we end up seeing how he has become a true hero in a compelling way.
In the season finale, Supergirl challenges Rhea to a battle to spare National City. Only for Rhea to prove herself untrustworthy. By redesigning a device her brother originally created to make the environment uninhabitable for Kryptonians, Lena was able to make the atmosphere toxic to Daxamites. And while this took care of the immediate problem but it also meant that Mon-El would have to leave the planet.
“Wherever I go, I’m going to be better because of you,” he told Kara during a tear-filled goodbye. “You’ll be in my heart.”
Kara replies, “You’ve made me so happy,” she replied, then gave him her necklace and finally dropped the L-bomb, acknowledging, “I should have said it before.”
The season premiere of Supergirl will be on Mon., Oct. 9 at 8 p.m. EST on The CW.