Lex Luthor enacts a devious plot to sabotage a scientific mission to the sun, intending to doom Superman to a fatal overexposure to solar radiation. Contrary to Luthor's expectations, the hero is not instantly vanquished but instead finds himself imbued with heightened powers and intelligence. While initially benefiting from this surge in abilities, Superman soon discovers the grim truth: his cells are over-saturated with energy, leading to an impending fatal condition. Amid this revelation, Superman embarks on a quest to accomplish as much good as possible with the time he has left, endeavoring to leave Earth with a lasting legacy of hope and justice.

As Superman confronts his mortality, his efforts are continuously challenged by a series of threats and superpowered antagonists, all while his relationship with Lois Lane deepens. He reveals his true identity to Lois, granting her temporary superpowers so that they may share an adventure together as equals. This poignant journey through Superman's final days is marked by a series of heroic and personal milestones, and as he attempts to prepare mankind for a future without him, the narrative weaves through moments of pathos, reflection, and confrontation with the complexities of his existence as both a hero and an individual facing the end.