Lois, Jimmy, and Perry are captured and forced to watch as Luthor kills Superman and ensures he's the real thing and definitely dead.
The story ends on a very bittersweet note, as while Supergirl is able to fulfill the same duties her cousin does and is thanked by the populace for carrying on his legacy, she takes no pleasure in the honor that's now hers.
While the comic was originally intended to be an Imaginary Story where the odds were a million to one that it'd never happen, later this story was retconned to take place on Earth-149, meaning in one corner of the multiverse it did happen.
Look at the title of this arc and put yourself in the time. Superman, THE codifier of the entire superhero genre. The Big Blue Boyscout. America's Number One Son. The character everyone was confident could save the day. Dies. Beaten to death in a Heroic Sacrifice by the monster literally called Doomsday. Popular culture was shocked at this event, with fans writing thousands of letters to DC begging them to bring him back and holding mock funerals for the Man of Steel. It was so large of a reaction is made multiple television and printed news stations for days.
Watching Superman's last blows with Doomsday, as his friends, colleagues, and family watch helplessly, is truly heart breaking.
Superman's last words are to ask if Doomsday's been stopped.
Poor Jonathan and Martha can't attend their son's funeral because they'll blow his secret identity. All they can do is stay in their farmhouse and torture themselves with grief. It's even worse when Jonathan has a heart attack and almost dies himself, especially the cover for the issue.
The entire Funeral For A Friend storyline, especially Bibbo Bibowski's reaction.
Bibbo: God? 's me, Bibbo. Been a awhile since we talked... I know my pal Superman is with you... So I guess he don't really need my prayers... But the rest o' us sure do. Take good care 'o Superman... Okay, God? I miss 'im... I 'spect just about ever'body misses 'im... God? I gotta ask ya... why? Why should Superman die when a washed-up ol' roughneck like me goes on livin'? It ain't right, God... It just ain't right.
Seeing Jonathan and Martha grieving over their son's death will break your heart. Especially since, unlike in the comics, they do attend the grand funeral in Metropolis. . . as just some of the thousands of citizens there to pay their respects and mourn their hero. Just faces in the crowd. And a police officer actually asks them to step back from the barricade separating the "commoners" from the VIPs.
Bibbo Bibbowski standing alone on the dock, lamenting how it's not fair that someone as good as Superman had to die while he, just an old sailor, gets to keep living.
While being civil, there still appears to be some tension between Cyborg and his dad.