SupergirlSave

Argo City — a surviving fragment of Krypton trapped in inner space — is the only Kryptonian community to survive the planet's destruction. Kara Zor-El (Helen Slater), Superman's teenage cousin, lives there with her parents Zor-El (Simon Ward) and Alura (Mia Farrow), and her mentor Zaltar (Peter O'Toole), the city's resident artist-philosopher. Zaltar shows Kara the Omegahedron, the artefact that powers all of Argo City; through his carelessness, Kara accidentally lets the Omegahedron escape through the city's wall and into Earth's atmosphere. The city begins to die without its power source. Kara, against her parents' wishes, follows the Omegahedron through the wall and arrives on Earth in Lake Michigan.

Earth's atmosphere transforms Kara: she emerges from Lake Michigan in a blue-and-red Supergirl costume identical to her cousin Superman's, complete with cape and S-shield. She tests her powers — flight, super-strength, heat vision, freezing breath, super-speed — by saving a construction site, lifting a runaway truck, and incinerating a downed power line. She discovers Earth gravity gives her abilities far beyond what she could do on Argo City. With the Omegahedron lost somewhere on Earth, she needs a base of operations; she enrols under the name Linda Lee at a Midvale girls' boarding school where she can hide in plain sight as a teenage student.

The Omegahedron has been found by Selena (Faye Dunaway), an aspiring witch living in a derelict Midwestern roadhouse with her sycophantic accomplice Bianca (Brenda Vaccaro). Selena recognisizs the artefact as alien technology with cosmic power. She uses it to begin granting her own wishes: she conjures a luxury mansion to replace the roadhouse, summons servants, and starts plotting to seize political power over the local town and eventually the world. Selena's witchcraft is presented as a Renaissance-grimoire tradition — she chants Latin incantations, draws pentagrams, and consults an old leather-bound spell book.

At Midvale Girls' Boarding School, Linda Lee meets her roommate Lucy Lane (Maureen Teefy) — younger sister of Lois Lane, who is mentioned but never seen on screen. The school is run by the brittle headmistress Mrs. Bigby (Sandra Dickinson) and includes a memorable boys' school across the way. Linda befriends Lucy quickly; the two have multiple sister-bonding sequences that lay the foundation for Lucy eventually discovering Linda's secret identity. The school sequences feature substantial comic interplay with the rivals' school's groundskeeper Ethan (Hart Bochner), whom both girls develop feelings for.

Ethan (Hart Bochner), the groundskeeper of the neighboring boys' school, becomes the film's romantic complication. Selena, on a drive through the area, sees Ethan and decides he is the perfect man for her — she conjures a magical love potion designed to make him fall for her instantly. The potion misfires; Ethan, drinking from the wrong cup, falls instead for the first woman he sees after consuming the potion — Linda Lee. Selena, enraged, dedicates herself to destroying Linda. The romance subplot is the film's primary emotional engine; Linda's confusion about her sudden popularity is comedic and substantive in equal measure.

Selena's first attack on Linda is the Invisible Demon — a hulking, semi-transparent monster Selena conjures from her own resentment. The demon attacks Midvale during a school dance; Linda transforms into Supergirl in a deserted greenhouse and fights the demon through the school's gymnasium, eventually defeating it by manipulating the demon's body through telekinesis. The sequence is the film's first major action set-piece; Slater's wire-work and physical commitment to the fight choreography earned widespread positive critical comment.

Selena retaliates by sending the Omegahedron's power to physically capture Kara. Kara is kidnapped to the Phantom Zone — the same dimensional prison Superman uses for his most dangerous Kryptonian villains. There, she encounters Zaltar (Peter O'Toole), her mentor, who has been exiled to the Zone for some unspecified earlier crime. Zaltar, broken and contrite, helps Kara understand her own powers and warns her that escape requires substantial will. The Phantom Zone sequences are the film's most-cinematically ambitious — featuring oversized rocky platforms suspended in starfield voids, mournful Peter O'Toole monologues, and Kara's gradual realisization of her own purpose.

Kara escapes the Phantom Zone with Zaltar's help and returns to Earth, determined to recover the Omegahedron and end Selena's threat. Their final confrontation takes place at Selena's mansion, which has been magically transformed into a Bavarian-castle stronghold. Selena summons multiple demonic entities — including a flying Demon and a fire-breathing creature — in a climactic battle. Kara defeats Selena by physically tearing the Omegahedron from Selena's grasp and using its power to imprison the witch in a dimensional pocket. The Omegahedron is restored to Argo City through a final flight back through Lake Michigan's portal.

Argo City is restored. Kara returns home, transforms back into her civilian Argo-City identity, and is welcomed back by her parents and the now-redeemed Zaltar. The film's epilogue suggests that Kara could one day return to Earth as Supergirl when needed; this never happened in the Salkind franchise, but Helen Slater would later return to the role in spirit through her recurring Smallville (2007-2011) and Supergirl (2015-2021) television appearances as Linda Lee's adult successor.