"Whatever Happened to the Man of Tomorrow" is a Superman story written by Alan Moore and drawn by Curt Swan, appearing in Superman #423 and Action Comics #583, in 1986. The stories were meant to be the final stories of the pre-John Byrne Superman, and in many ways recaptured the storylines and atmosphere of earlier, Silver Age comics. It is however, somewhat darker than those comics, but nowhere near as dark as some of Moore's other work.
The story is told through flashbacks, as a reporter interviews Lois Lane, now a married housewife, about the last days of Superman. She recounts how Superman's old enemies, once somewhat comical menaces, started attacking him in earnest. When his Clark Kent identity is unmasked (so to speak), and his enemies attack the Daily Planet, he takes his closest friends and to the Fortress of Solitude, where they prepare to withstand a siege from the heavyweights. The Legion of Super Heroes visits from the future to give Superman a trophy, and their appearance makes it obvious to Superman that what is about to happen is very major.
In the final issue, the siege begins, with a barrier preventing Superman's allies from helping him. Jimmy Olson and Lana Lang regain their Silver Age powers to fight the supervillians, but perish in the process. Krypto the Superdog soon dies as well, and in the penultimate battle, Superman's foes scatter. He realizes that only one enemy has not shown up, the enemy that actually orchestrated the attack---the once playful imp Mxyzptlk, who now reveals himself to be a much darker character. Superman, using a hint from the Legion of Super-Heroes, banishes Mxyzptlk to the Phantom Zone, but accidentally kills him in the process. Remorsefully, he exposes himself to Gold Kryptonite, losing his powers, wanders out into the snow, and presumably dies. The last page of the story reveals that he did not die, and is actually Lois Lane's new husband in disguise.
The plot description I gave was both unnecessary and too short. The important thing in the story is that Alan Moore manages to tie together all of the major threads from the Superman mythos together in these two issues, and write what is one of the most perfect examples of a Superman story. I also find it good that Alan Moore can tone himself down for this story: while it is recognizable as an Alan Moore work, with a dense plot, several turns, and many jokes and references, he dovetails his own style with the fact that he is writing a homage to the Silver Age Superman. It is a great story and another pillar in Moore's Body of Work.