The Pocket Universe is a singularly created universe that was developed by the Time Trapper as part of his scheme to control the Legion of Super-Heroes after the Earth-One Superboy was wiped from existence due to the effects of the Crisis on Infinite Earths and replaced with the current Superman who never became Superboy. This change threatened to destroy the 30th century Legion of Super-Heroes' existence and all who populated it as it was the superhero efforts of the Earth-One Superboy who inspired the Legion in the first place, creating that reality. Unable to directly access the "Dawn of Time" and change the time stream back as it was before the Crisis, the Time Trapper used his massive control over the resulting time stream to create a small universe that reflected the Pre-Crisis Earth-One reality of Earth and Krypton to a very close level. With that Earth created and Superboy developed almost exactly as in the Earth-One reality, the Time Trapper routed the Legion's time travel machines so they would always be shunted into the Pocket Universe Earth's history rather than their own Earth which was now reformated.
Unlike the Earth-One universe this pocket dimension only had two populated planets, Earth and Krypton. The Time Trapper deemed only these two necessary to recreate the now missing Earth-One and its Superboy. This would explain why no other powered heroes who were dependent on extraterrestrial influence developed on this particular Earth as in other parallel realities.
After establishing himself for several years as a hero and forming the Legion in line with the pre-existing Earth-One incarnation, the Time Trapper reveals him to this Superboy the problem with reality, convincing Superboy to aid him in capturing and killing the Legion, or else all reality be destroyed. When the Legion arrives in Pocket Universe Smallville, they are attacked by Superboy who follows them to the new Primary DC Earth, the home of the current Superman.
Thinking they have only jumped a few years forward and not to another entire separate reality, the Legionnaires try to contact Superman, thinking that he is merely an older version of their friend and would explain his earlier actions. When they confront Superman, his powers accidentally activate, leading to the Legion attacking him until Brainiac 5 forces the fighting to stop, whereby discussions can be revealed. It is during the discussions that Superboy arrives and freezes everyone with his immobilizer ray. Superboy loads his four frozen former friends into a time bubble and leaves Superman behind after correctly stating that the adult Kal-El is not the Superman who he would grow up to become. After a few moments Superman begins to regain his mobility and immediately sets off in pursuit of Superboy and the still immobilized Legion members.
Superboy, faster and far more powerful than Superman, prepares to enter the time stream under his own power and leave the primary DC Earth behind. Superman begins to falter behind due to Superboy's vastly greater speed but manages to grab hold of Superboy's booted foot before entering into the actual time stream and is dragged along by Superboy to Superboy's world. Superman comments on how bad the time travel is to him and how impressed he is by the speed that Superboy has at his control that allows him to break the time barrier on his own speed. Superman ultimately loses his grip and falls off the time stream and crashes into ground not far outside of Pocket Universe's Smallville, where he is found by the younger Pocket Universe Pete Ross, who Clark mentally notes to look only similar to the high schoolmate he once knew, showing the differences from the new Earth history and that of the Earth-One version.
As the Legion attack, the Trapper reveals his hold over Superboy to the assembled Legion members, "This never was your past, Legionnaire. Only a whispery possibility I connected you to when you were presumptuous enough to try to travel through time. Each time you 'broke' the time barrier, I sent you here ... until this Earth began to fall apart and the jest paled." With that the time Trapper damaged the machinery that held back the destructive energies of the Crisis and left. Once again the red skies began assaulting the Pocket Universe. As the heroes tried to repair the machine, Superboy realized that the only way to repair the machine is to find some way to channel its energies. Lacking the time to find proper insulators to the enormous energies needed to safeguard an entire universe, Superboy jumps into the machine to act as the insulator and allows the universal energies to pass through him. He endures until the pocket universe is moved away from energy waves of the Crisis to some unknown safe locale in the cosmos.
With the world safe, Superboy finally allows himself to collapse. Ma and Pa Kent watch in horror as the Legionnaires try to save the injured Superboy by taking him back to the 30th century. But arriving in the safeguarded future, Superboy dies from his injuries.
It would be revealed later in Superman Vol 2 #21 that Superboy was never taken back to his native time but buried in the 30th century though was missing for so long that the Kents gave him a funeral at which time they revealed to the world at large that their son Clark was Superboy. These Kents would die of heartache and grief not too long after that time leaving the Pocket Universe to be defended by its Luthor and Supergirl (the shapeshifting Matrix) who would become the sole survivor of this pocket reality after the destruction done by that reality's General Zod and henchmen.
In the 30th century the Earth of the Pocket Universe was pulled into the New Earth reality to replace the destroyed Earth. However it turned out that the Pocket Universe had slightly different physical laws causing the planet to effectively turn into a giant bomb.
- This reality was identified in Crisis on Infinite Earths: Absolute Edition as Earth-Twenty-Three even though the Time Trapper created it rather than saving it from the Pre-Crisis reality.
- The Pocket Universe was created as a way to maintain Legion of Superheroes stories that contained Superboy in them, since the Superman of the New Earth reality created after the Crisis On Infinite Earths with John Byrne's Man Of Steel reboot never adopted the identity of Superboy.
- While the Pocket Universe explained the existence of Superboy, it did not explain the existence of Mon-El.
- The very existence of the Pocket Universe invalidated Harbinger's claim that there was only one Earth, one history.
- Due to the changes brought on by Infinite Crisis Superman once again was a member of the Legion of Super-Heroes when he was a Superboy eliminating the need for a Pocket Universe. However, the Time Trapper remarks how he tried to confound the Legion with pocket dimensions and Batman states "I've met three different Legions, Clark" (sic) and references events from Earth-One and Infinite Crisis New Earth continuity raising the question if the Pocket Universe still existed in post Infinite Crisis continuity.