In this spin off from the Christopher Reeve 'Superman' franchise films, teenager Kara Zor-El (Helen Slater) lives in the isolated Kryptonian community of Argo City, which was transported into another dimension, called "inner space," when the planet Krypton exploded many years ago. The city is powered by a paper-weight-sized orb known as the "Omegahedron." Argo City resident Zaltar (Peter O'Toole) secretly borrows the Omegahedron for an art project, but a mishap hurls the orb into space. Argo City cannot survive without the Omegahedron, and Kara boards a single-passenger transport ship to retrieve it, much to the distress of her parents, Alura (Mia Farrell) and Zor-El (Simon Ward).The Omegahedron goes through a wormhole and lands on Earth where it is retrieved by Selena (Faye Dunaway), a carnival fortune teller who wants to become a witch. Selena, who has been studying magic under the warlock Nigel (Peter Cook), realizes the orb can assist her in casting spells and stores it away. She returns to the carnival where her long-suffering assistant, Bianca (Brenda Vaccaro), worries about getting the money to cover their bills, but Selena reports their financial problems are over and that she has plans for "world domination."Meanwhile, Kara is transported through the wormhole and arrives on Earth, transformed into "Supergirl," complete with a costume similar to her cousin, Superman, who is away in outer space on a peace-keeping mission. Kara discovers she has super-powers and flies around, amazed at the new world. Kara's bracelet glows whenever the Omegahedron is in use, and follows the beacon to Midvale, Illinois. There, Kara enrolls in an all-girls school, claiming to be Clark Kent's first cousin, an orphan named Linda Lee. The headmaster, Mr. Danvers (David Healy), assigns her to be roommate to Lucy Lane (Maureen Tuffy), the teenage sister of Daily Planet reporter Lois Lane.Selena and Bianca are both impressed when they notice Ethan (Hart Bochner), the hunky groundskeeper at the school. Selena concocts a love potion and slips it into Ethan's beer. Although the potion is supposed to make him fall in love with the first person he sees, instead, it makes Ethan dizzy, and he wanders through the town disoriented. With the help of the Omegahedron, Selena casts a spell causing a bulldozer to scoop Ethan up and bring him to her, but the unmanned bulldozer runs amok, wreaking havoc throughout town. Lucy Lane and her boyfriend, Daily Planet photographer Jimmy Olsen (Marc McClure), try to stop the bulldozer, but to no avail. Kara changes into Supergirl and stops the bulldozer, then changes back to Linda Lee just as Ethan regains consciousness. He gazes at Linda and instantly falls in love with her and kisses her.Observing all of this through a magic mirror powered by the Omegahedron, Selena is angered and vows revenge, casting a spell to capture Linda. As a dark cloud comes for her, Linda changes to Supergirl, takes a street lamp post and flies into a thunderstorm where the pole is struck by lightning. This creates a lightning rod that repels Selena's dark cloud. Because Selena used the Omegahedron to create the cloud, Supergirl's bracelet glows. Changing back to Linda Lee, she goes to the carnival to investigate, but finds Ethan there with candy and flowers for her. The two chat while sitting on a stopped carnival ride, but Selena casts a spell causing the ride to race at top speed. Ethan is knocked unconscious while Linda changes into Supergirl to confront Selena. Supergirl throws poles to surround Selena in a make-shift cage, then rescues Ethan.Having freed herself, Selena asks for warlock Nigel's help to defeat Supergirl, since he is more experienced casting spells. Nigel brings her a special magic wand that, when combined with the Omegahedron, creates powerful spells. In doing so, Nigel accidentally breaks the love potion spell, causing Ethan to fall out of love with Linda. However, Ethan meets Supergirl and is soon enchanted by her. Supergirl kisses Ethan just as Selena casts a spell teleporting Ethan to her lair. Selena also casts a spell creating a mountaintop castle just outside town. Supergirl flies there and finds Ethan in chains. Selena casts a spell banishing Supergirl to the Phantom Zone, the dimension where planet Krypton sends its criminals.Although she does not have any super-powers in the Phantom Zone, Supergirl is reunited with Zaltar from Argo City, who exiled himself to the Phantom Zone as penance for losing the Omegahedron. Zaltar insists there is no way out, but Supergirl believes they must look for one. Supergirl and Zaltar are forced to scale a large cliff to find an opening, but Zalter sacrifices himself to allow Supergirl to make it to the top to escape.Meanwhile on Earth, Selena has begun her plan for world domination. She rules Midvale with armed soldiers, but faces protesters, including Lucy Lane and Jimmy Olsen, whom she takes hostage.Supergirl breaks out of the Phantom Zone through a magic mirror and comes to Selena's castle where the witch casts a spell creating a large demon to attack her. The demon is on the verge of defeating Supergirl when she hears Zaltar's voice urging her to fight on. Supergirl breaks free and Nigel tells her the only way to defeat Selena is to turn the demon against her. Supergirl complies and creates a focused whirlwind that traps Selena, who is then attacked and incapacitated by the demon as the whirlwind pulls in Bianca as well. The three are sucked through the mirror portal, which promptly reforms, trapping them all forever in the Phantom Zone.Ethan retrieves the Omegahedron and gives it to Supergirl, who he now realizes is also Linda Lee. Supergirl asks Ethan, Lucy, and Jimmy not to say anything about her existence, then tells them she must return the Omegahedron. Selena's castle disappear as Supergirl flies back to Argo City through the underwater wormhole with the Omegahedron. In the final shot, Kara returns to a darkened Argo City, which then lights up again.