Deep in space, in Krypton's own solar system, Superman flies by in his space ship. Krypton is now nothing more than an asteroid belt of Kryptonite floating in space but Superman still checks for signs of life. There are none. However, the ship picks up a distress signal from another planet in the system. Feeling hopeful, Superman heads out to answer the call.
Superman follows the signal to a frigid world at the edge of the system. Finding that the planet is devoid of Kryptonite radiation, he investigates further. He finds a lab buried in the ice and snow. Inside, he discovers a recorded message from Kala In-Ze, the planet's former chief physician: the planet is called Argo and was once a thriving civilization. But when Krypton exploded, Argo was ravaged by shock waves and torn from its orbit. Eventually, it moved so far from the sun that all inhabitants froze to death. To survive, Kala placed herself and her family in stasis chambers in hope of rescue.
After the message finishes, Superman finds the chambers have all been destroyed by weathering - except one, holding a teenaged girl. 3 weeks later Back on Earth, the girl, Kara, flies over Kansas reveling in her newfound powers. Unfortunately for her, Clark grabs her and pulls her down, scolding her for being so reckless. He reminds her that she needs to keep a low profile. She agrees, but complains about the fact that he won't let her fight crime along with him. He tells her to be patient.
Over lunch with the Kents, Jonathan mentions that Intergang is causing trouble in Metropolis again. Kara is immediately interested, but Clark insists that she need not worry. As soon as he leaves, however, she presses Jonathan for more details.
In Metropolis, Lois and Jimmy are covering an electronics show, which is attacked by a couple of teenagers riding on flying disks and wielding beam weapons that shoot fire and ice. Superman arrives and confronts the duo but their weapons are strong enough to keep him at bay. He manages to capture one of the teenagers but the other grabs Lois and sticks a flying disk on her, sending Lois shooting into the sky and distracting Superman long enough for them to escape.
The teenagers return to their gang's hideout, in the Hobbs Bay recreation center, where they are greeted by Granny Goodness.
Meanwhile, at the Daily Planet, Kara arrives disguised as "Karen," Clark's cousin, wanting to help him. Clark insists that she stay out of the case and sends her back to his apartment. At the same time, Jimmy complains to Lois that he wants to join in on the Intergang investigation. He insists that he has a lead, a video arcade token that one of the teenagers dropped. Lois refuses to let him tag along, and Jimmy decides to check the arcades himself. Kara overhears, and, seeing her chance, decides to join him.
The pair eventually finds the right arcade and discover the same two teenagers that fought Superman. They follow them to Intergang's hideout and sneak in. Unfortunately, they are caught. Even though Jimmy claims that they only want to join, Granny decides to have them beat up. Kara decides she's waited long enough and tells the thugs that Granny is just using them. Angered, Granny orders Kara killed but she easily beats the group and unveils her costumed identity: Supergirl. Granny starts attacking with her own weapon but Kara manages to destroy it. Not ready to call it quits just yet, Granny opens up a boom tube and summons three of her Female Furies. Together, Lashina, Mad Harriet and Stompa attack Supergirl but her fighting prowess is greater than they thought. She manages to defeat Lashina and Mad Harriet. Unfortunately, Stompa manages to force Supergirl out of the building. The gang members run out leaving Supergirl to fend for herself.
Meanwhile, Clark listens to Dr. Cornell telling about a comet that will be coming within a couple million miles of Earth. Soon after, the ground begins to shake and Clark notices the fight. He goes to aid Kara but during the ensuing fight, she is buried under a fallen building. Superman attempts to save her but the Female Furies take advantage of his distraction and capture him. Superman is taken to Apokolips, put into restraints, and tortured.
In the back of the gang's hideout, Supergirl, Jimmy, and Amy investigate Granny's inner sanctum and find a large machine, built from technology stolen by Intergang. None of them knows what it does, but Supergirl finds a boom tube generator dropped by Granny and heads off to Apokolips herself. She is appalled by the place but keeps up her search to find Superman. Twice she is nearly captured, first by a couple of guards mounted on war dogs, and then by a troop of Parademons. Supergirl runs from the parademons and loses her boom tube generator. Still unable to evade them she finally manages to incinerate them using fire from a fire pit. Shortly after escaping, Supergirl witnesses Superman being dragged away to Darkseid's palace.
Superman is brought before Darkseid, who explains that since he cannot conquer the Earth, he'll destroy it. Granny was sent to Earth to create a device capable of pulling the comet into an impact with Earth. Angered by her memories of Argos, Supergirl leaps into the fight, but Darkseid quickly disables her with his Omega Beams. Fortunately Superman manages to hit Darkseid and force him to stop, but Granny soon disables him.
The Furies attack Supergirl, and in her weakened state she is nearly defeated. But her fighting prowess shows again, however, as she uses Stompa's attack against Mad Harriet, then defeats Stompa. She then faces off with Lashina, who nearly defeats her single-handed, but her lash accidentally wraps around Granny's weapon, disabling them both. Supergirl frees Superman and the pair escape into a boom tube generated by Lashina's generator.
Returning to Earth, Superman and Supergirl see the Doomsday Magnet, which Amy and Jimmy have tried unsuccessfully to destroy or deactivate. The comet has already been pulled onto its impact course. Supergirl destroys the Doomsday Magnet despite Superman's warning, and he scolds her, since he could have used it to push the comet away. Now he'll have to do it the hard way.
He flies into the upper atmosphere to push it away. He manages to do so, but a chunk of it breaks free and heads straight for Metropolis, large enough to cause massive devastation. Realizing it's up to her, Supergirl hurtles at the chunk of comet, destroying it. Superman finishes pushing the comet away and flies down to catch the falling Supergirl, knocked unconscious.
On Apokolips, Darkseid orders the Furies to torture Granny as punishment for her failure.
At the Planet, Jimmy proudly admires his front-page story on Supergirl's debut. As his co-workers eagerly crowd around for more details, he points out the window, where Kara flies around Metropolis, proud of her achievement and confident that Earth has a new heroine.
