When Superman fell through a black hole which transported him back to Krypton before its destruction, he finds a way to save it from certain doom. The result is a contaminated timeline where drastic changes are made to history.
When Superman returns to the present, he finds that in this new reality, he never came to Earth to become a superhero, because he never left Krypton. Because of this, the Justice League of America is not as powerful as they were in the reality that Superman remembered, and they were killed by the Legion of Doom. All but Robin that is, but he didn't stand a chance against the Legion of Doom all by himself.
The world was now in a state of total chaos due to the Legion of Doom's reign of terror. Realizing that saving Krypton was a mistake, Superman grievously returns to the past and stops his counterpart from saving Krypton, thus restoring the original timeline.
Lost Episodes (1983):