As six months have passed since Superman’s brave sacrifice against the monstrous Doomsday, a palpable unease has taken hold of Metropolis. The once vibrant streets are now overwhelmed by rampant crime, and in the aftermath of Kal-El's departure, four self-proclaimed Supermen emerge, each vying to inherit his legacy. Lois Lane, still grappling with her crushing grief, dedicates herself to investigating these newcomers while meticulously analyzing intelligence gathered from various sources.
In the midst of this chaos, the cunning Lex Luthor orchestrates a high-profile press conference, boasting his latest creation: Superboy, a genetically engineered clone developed by the enigmatic Dr. Dabney Donovan. However, Lois quickly uncovers the deception at play, revealing this so-called Superman to be nothing but an artificial copy. The situation escalates when a trio of other Supermen – Steel, Cyborg Superman, and the Eradicator – storms in, resulting in a dramatic showdown that leaves only two of them standing.
Unbeknownst to everyone, Kal-El has managed to cheat death and is recuperating in his secluded Fortress of Solitude. As he grows stronger, it becomes evident that the Eradicator’s DNA has started to undergo rejuvenation thanks to the ship's advanced matrix technology. That same evening, Cyborg Superman confronts Lois, attempting to convince her of his legitimacy.
As tensions simmer among all involved, the Justice League, with the support of Superboy, has been appointed as bodyguards for the President during a paramount event meant to unveil the new Watchtower. Just as the ceremony is about to commence, it is abruptly interrupted by an horde of Parademons flooding in through a boom tube portal. To make matters worse, the League members are mysteriously transported to another planet and presumed dead, enabling Cyborg Superman to leap into action, rescuing the President and declaring himself the true Superman.
In the subsequent chaos, Lex Luthor's frustration with Superboy mounts to a breaking point, igniting a chain of violent events culminating in the horrendous murder of Dr. Donovan—a heinous act that he apparently orchestrated, leveraging his genetic connection to the clone. Meanwhile, a ferocious conflict unfolds between the Eradicator and Steel, which culminates when Lois realizes that this supposed Superman is merely a holographic projection conjured by Kal-El's ship’s matrix to protect his hidden truths.
As the dust settles, a shocking revelation comes to light: Cyborg Superman, in reality Hank Henshaw, a fallen astronaut consumed by a drive for vengeance against the Man of Steel, has been employing Mother Box technology to construct an army of human-infused warriors known as the Cyber Corps. The warning is clear—if left unchallenged, these fierce forces are hell-bent on obliterating everything that Kal-El stands for.
Ultimately, Lois and Lex stumble upon a harrowing secret: Cyborg Superman is being manipulated by none other than Darkseid, the tyrant from Apokolips, who set the stage for Doomsday's assault on Earth, aiming to eradicate the Last Son of Krypton once and for all. As Superman, Superboy, and Steel rush back to Metropolis, Lex Luthor seizes the chance to save the Justice League from their impending doom. On the other hand, Darkseid fixates on tormenting Henshaw, whose failure to eliminate Superman still writhes in his conscience. Using a Mother Box infused with Henshaw's consciousness, Darkseid tries to create a portal to manifest his enemy. However, before his plan can come to fruition, Henshaw takes matters into his own hands, forcibly removing the Mother Box from within himself, consequently breaking Darkseid's grip on the Cyber Corps.
As Lois Lane finds herself whisked away to the Watchtower, she becomes the target of Henshaw's fury. Just as he preps to deliver his vengeance, Superman makes a dynamic entrance, only to face an onslaught of rage and resentment from Henshaw, who blames him for the death of his wife, Terri, resulting from Superman's past actions. Ultimately, the Man of Steel draws strength from the sun's rejuvenating rays, bringing an end to Henshaw's rampage with a crystal that houses the Eradicator program, acquired from the Fortress of Solitude.
Following this climactic battle, the Martian Manhunter steps in, masquerading as Superman to announce the safe return of Clark Kent, conveniently glossing over his disappearance during the catastrophic Doomsday showdown. Meanwhile, Superboy is embraced by Jonathan and Martha Kent, who lovingly rename him Conner as they integrate him into their family. In a tantalizing post-credits scene, the Justice League reconvenes at the Watchtower, preparing to confront Darkseid directly, while Lex Luthor extends an unexpected offer to join forces, hinting at future alliances.
