This episode is one of the best adaptations of "The Death of Superman" story from the comics ever made.
With Superman believed dead, the entire world mourns his death. The episode is superb, featuring a lot of character development: Batman is the only one who doesn't want to believe Superman's dead and tries to do everything he can to find out the truth. Flash admits he always acts the way he does because he knows Superman is behind him, protecting him. Shayera flies to the mountains where she cries because of his death. Even Lex Luthor admits he never wanted this to happen this way.
A sad episode, but overall an enjoyable one. Be sure not to miss the second part for more answers about Superman's "death".