After being exposed to a massive dose of solar radiation while saving a space mission, Superman discovers that his cells are beginning to deteriorate and that he has little time left to live. Faced with this fate, he decides to complete a series of tasks he considers essential: protecting the people he loves one last time, revealing truths he has long kept hidden, and preparing the world for his absence. While Lex Luthor pursues his own plans, Superman faces several threats as he tries to give meaning to his final days and to the legacy he will leave behind.