The second part of the episode Hereafter is completely different from the first. This episode reveals that Superman is not really dead, but in fact transported away to a future, desolate world that he later realizes is Earth. It focuses on Superman surviving in the wilderness for perhaps months as he journeys toward a Justice League com-link signal. Superman's resourcefulness leads us to believe that he may actually have been a boy scout.
Superman meets a very different Randal Savage, who as we know is immortal. Having survived for the past 30,000 years, Savage has learned to cope with the fact that he was responsible for killing everyone on Earth. His character is remarkable, having become a philosopher in his countless millenniums of life. He has realized that his quest to rule the world was pointless, and instead tries to rebuild what is left.
Savage had, at one point, tried to make a time machine, but what he could build with his limited resources only allowed a limited type. He could only travel to a point in the past where he did not already exist. However, Superman was already dead in his present, so he would happily make the journey. He and Savage work together to build the machine. Superman goes back and prevents Savage of the past from destroying Earth. Savage sees the Earth from 30,000 years in the future as it would have been if he had not destroyed it, and thanks Superman just before disappearing from existence. (It's pretty optimistic, if you ask me, assuming that humans will still be alive so far in the future.)
This is a superb episode, the finest example of the kind of creativity and genius I have come to expect from Timm and the other guys who continually make exceptional work for the DCAU.
I especially love science fiction-type shows, which are captured well in just about every episode of Batman Beyond. But my favorite superhero will always be Superman, and giving him this awesome time-travel storyline resulted in an epic, classic story. My favorite of the whole Justice League Series, which deserves much more praise than it has received.