The movie is an animated adaptation of the popular comic book storyline of the same name. After the death of Superman, the world is in shock and new heroes emerge to protect it. The Last Son of Krypton is a mysterious figure who claims to be Superman returned from the dead, but his powers and motives are unclear. Superboy is a clone of Superman created by the villainous organization Cadmus, who struggles to find his place in the world. Steel is a man named John Henry Irons who creates a suit of armor resembling Superman's to fight crime. Finally, the Cyborg Superman claims to be the real Superman returned with new cybernetic enhancements. Throughout the movie, each of these new heroes faces their own challenges and struggles with their newfound responsibilities.
Meanwhile, a new threat emerges in the form of the vicious alien tyrant Mongul, who sees Earth as vulnerable in the wake of Superman's death and seeks to conquer it. The Justice League and a host of other heroes band together to stop him, but struggle in the absence of Superman's leadership. As the four new Supermen fight to protect the world and prove themselves, they must also contend with the legacy of the original Superman and the doubts of those who loved him. In the end, they must all come together to stand against Mongul and protect the Earth from destruction.