In Argo City on Krypton, Alura Zor-El has been preparing for a catastrophe her brother-in-law Jor-El has predicted will destroy the planet. However, the disaster strikes early, and Alura has just enough time to send her daughter Kara to Earth, where she is supposed to reunite with her younger cousin Kal-El, who is to arrive in his own escape ship. Just after take-off, a fragment of the exploding planet damages Kara's pod, putting her into suspended animation and sending the vessel into a trajectory that arrives on Earth later than intended.
After her arrival, Kara - now named Supergirl - has trouble adjusting to life on Earth with its more primitive culture, and the superpowers gained from her new environment. During a fight with Solomon Grundy, she spots masked individuals lurking nearby and is rescued by Kal-El (now called Superman) and Batman. Kal-El suggests that Kara should travel to the 31st century and join the Legion Academy, a training school for Legion of Super-Heroes candidates. In the 21st century, Batman intercepts the masked individuals raiding S.T.A.R. Labs, but they commit suicide before he can question them.
Traveling to the future, Kara is introduced to the Academy by one of its trainees, Mon-El, but starts a spontaneous fight with Academy member Brainiac 5, whom she mistakes for her cousin's enemy Brainiac. While the misunderstanding is cleared, Brainiac 5 is distrusted because of his ancestry. Kara's own relationship with him is initially antagonistic as well, but as time passes, they develop a mutual understanding which eventually turns romantic.
One night, while looking for school pet Proty, Triplicate Girl spots Mon-El lurking near a forbidden high-security vault on the Academy's grounds, before being attacked and apparently killed. The Legionnaires arrest Brainiac 5 and discover that he enrolled in the Academy to penetrate the vault, which contains dangerous weapons. Brainiac 5 explains that he is trying to counter an organization called the Dark Circle, but only Kara believes him. After recognizing the masked figures she saw in the past from the Academy computer's files on the Dark Circle, she breaks Brainiac 5 out of confinement and learns that the Dark Circle is trying to obtain the reality-bending Miracle Machine.
Joined by Mon-El, Kara and Brainiac 5 enter the vault and overcome its security. When they find the machine, Mon-El reveals himself as an agent of the Dark Circle, stabs Kara with a kryptonite blade, and summons the rest of the Circle who overpower the Academy teachers and students. The Circle's leader appears and is revealed as Brainiac, who founded the Dark Circle in the 21st century to facilitate his resurrection[a] and has plotted for Brainiac 5 to grant him access to the Miracle Machine. Because they were incapable of passing the vault's security, the bodies of Brainiac 5's predecessors were melded into a new body for Brainiac. He claims that he has foreseen great danger approaching, and plans to bend reality to eliminate it and make himself the savior of the universe.
Kara and Brainiac 5 are rescued by fellow students Triplicate Girl (who had lost merely one of her alternate selves), Invisible Kid and Phantom Girl, who have eluded capture, and together they retrieve Dawnstar, Bouncing Boy, and Arm Fall Off Boy. After sending an SOS to the Legionnaires in space, the other students distract Mon-El and the Circle while Kara and Brainiac 5 move to stop Brainiac. They cannot prevent the Miracle Machine's activation, but Brainiac 5 turns his predecessors against their creator, causing Brainiac's body to tear itself apart. The machine goes out of control, and in her attempt to stop it, Kara summons Alura's soul. Though briefly tempted to reverse Krypton's destruction, she comes to terms with her loss and stops the cataclysm by displacing the machine into another reality. Afterwards, the students defeat Mon-El and the Circle, and the returning Legionnaires grant them all full Legion membership.
In a post-credits scene, Kara contacts Superman about her decision to stay in the 31st century and her new relationship with Brainiac 5. After the call is ended, Superman and Batman inspect a crater in Metropolis when they are hit and apparently annihilated by a beam blast.[b]
The idea of a Legion of Superheroes film was conceived by Butch Lukic and Jim Krieg, who wanted to incorporate both Supergirl and the team into the Tomorrowverse.[3][4] Lukic hired Jeff Wamester as director due to their previous collaboration in Justice Society: World War II (2021).[4]
Donnelly was cast as Supergirl after voice director Wes Gleason showed Lukic footage of her previous work.[4] Robinson was allowed to improvise many of his lines during recording. He also wanted to develop his own take on Invisible Boy, as the film marked his first appearance in media outside of comics.[9]
Release
On November 10, 2022, James Gunn confirmed that all DC projects will be released under the DC Studios banner which includes animated productions. And on February 2, 2023, Gunn confirmed that they are being released and labeled as DC Elseworlds.[8]
On the review aggregator website Rotten Tomatoes, 60% of five critics' reviews are positive, with an average rating of 5.3/10.[11] Sam Stone of Comic Book Resources wrote: "Though the movie could up the ante on some of its major set pieces, it does get to the heart of what makes the Legion tick".[12] Brian Costello of Common Sense Media described the film as "a pretty standard but enjoyable superhero animated action movie".[13]