Zero Hour

Zero Hour: Crisis in Time sort of marks the 10th anniversary of Crisis on Infinite Earths.  While the intent of Crisis was to simplify DC's complex multiverse into a single universe, the purpose of Zero Hour in retrospect seems to be to clean up many of the continuity problems which had unintentionally arisen because of Crisis.

Zero Hour was a 5-issue mini-series which had the innovative 90's gimmick of starting at issue #4 and counting backwards to #0.  Every DC book will also have a tie-in zero issue the following month.  Zero Hour's repercussions would be felt throughout the DCU for the rest of the decade and would define much of the 90's.  Many of its changes would prove to be unpopular with fans and would eventually be undone the following decade.  Former Green Lantern Hal Jordan, who went insane after the fall of Coast City, would be revealed as the big bad of Zero Hour, trying to re-make the universe in his image.  The Legion of Super-Heroes would get its first real hard reboot, and the team that got their name and inspiration from the Man of Steel would unfortunately no longer be in the Superman family in any sense until after Infinite Crisis.

Superman: The Man of Steel #37

Multiple Batmen come to Metropolis to alert Superman of an anomaly in time.  The first one looks Neal Adams' depiction from the 70's, followed by one resembling Frank Miller's Dark Knight, and lastly one resembling Bob Kane's original version. 

Superman hesitates to leave Metropolis again, especially while the benefit concert for the city is happening.  Jeb's union is sponsoring the production, and he once again tries to steal Lois away from Clark.  Jimmy's ex-roommate Babe, now a big star, performs in some ghoulish vampire-like make-up and appears to turn into a giant on stage.  No, it's not really make-up and special effects, this story line will continue a few months later. 

The other Batman vanish and then the one from his own universe arrives in Metropolis to find Superman, where they're both found by Metron of the New Gods in a page virtually identical to a page from Zero Hour #4.  Meanwhile in Smallville, a Kryptonian ship lands at the Kent farm and very much alive Jor El and Lara are looking for their son.

Superman #93

In Zero Hour #4, Metron projects a message from Superman to virtually all the heroes in the universe.  We get the exact same message here, but instead of switching to a different hero each panel we see the whole thing through Supergirl's point of view.  Before Superman can join the other heroes in the crisis in time, he gets a call from his parents and rushes to Smallville where he's shocked to see his Kryptonian parents.  This alternate version of Jor El and Lara managed to save Krypton after they sent their son away, and have come to Earth hoping to bring him back with them.  Superman turns down their offer, and their ship (which resembles Kal El's Kryptonian birthing matrix and looks nothing like the ship depicted on the cover) is destroyed in a chronal discharge as they try to go back to Krypton.  Although Clark knows they're from an alternate reality, he nevertheless feels orphaned again by this experience.  He collects his composure, makes sure the Kents are alright, and then joins Metron into the timestream to save the universe.

Adventures of Superman # 516

Superman comes back to check on Metropolis and finds it magically rebuilt, but unfortunately nobody there knows who Clark Kent is, including Lois Lane and the rest of the staff at the Daily Planet.  Superman realizes that he's now in an alternate reality himself, a version of Metropolis whose champion is a hero named Alpha Centurion.  We learn that Alpha Centurion was a Roman named Marcus Aelius who was taken to an alien world and returned to Earth in his lifetime but centuries later due to time dilation.  Centurion's origin is nearly identical to Valiant Comics' X-O Manowar who was popular at the time.  This timeline's Lois Lane also named and became romantically involved with Centurion.  Feeling uncomfortable in this situation, Superman decides he can't stay in this timeline and he and Alpha Centurion fly off to set things right in the pages of Zero Hour.

Action Comics #703

The heroes believe they've stopped time from dying at both ends in the pages of Zero Hour, but back in Metropolis Superman realizes they were mistaken when elders like Perry White start to fade from existence.  Thinking of Ma and Pa Kent, Superman races back to Smallville, where reality shifts and he encounters younger versions of his parents who instead found a dead infant when his ship crashed to Earth.  Superman helps the Kents stop the evil Dr. Vale, who it turns out had left the infant Kal El to die.  But ultimately Superman is unable to stop both version of the Kents from disappearing from existence.  Liri Lee of the Linear Men pulls Superman into the timestream to enlist his help in fighting the real threat behind Zero Hour.  Back in Metropolis, Lois Lane watches as the city is consumed by the void.  She whispers to Clark that she loves him (wherever he is) as everything fades to white.  The last 3 pages of the book are totally blank, a recurring device in Zero Hour tie-ins, as well as other reality-ending stories in the 90's.