I used to love the movie Supergirl as a child and recently re-watched it. Unfortunately, it does not stand up well over time. However, I did discover it is a Girls Underground story, of the reverse type where the girl comes from someplace magical and enters the mundane world. (Other examples include Kiki’s Delivery Service, Princess Nevermore, Except the Queen, and the backstory of Pan’s Labyrinth.)
Kara lives in a Kryptonian city isolated in trans-dimensional space. When the power source of the city is accidentally sent out into space, Kara impetuously follows it on its trajectory to Earth in order to locate and return it to her city. Somehow, she magically transforms into Supergirl once arriving on our planet. She assumes the secret identity of Linda Lee when walking amongst people, and befriends a human girl named Lucy.
The power source, however, is first found by the evil Selena and her minion Bianca. Selena uses it to perform magical spells and control people. She tries to enslave a man she desires, but the spell goes awry and he falls for Kara instead. After battling a bit more, Selena manages to send Kara into the Phantom Zone – a desolate land much like an oubliette. But with the help of her old friend Zaltar, she escapes and confronts the witch who has made herself a ruler of Earth. She defeats Selena, sadly leaves her love behind, and brings the power source back to her city.