Luthor's plan to force Superman to kill Batman by threatening Clark's mother is foiled by them teaming up. Batman goes to rescue Martha, while Superman confronts Luthor, who reveals he's created Doomsday, a Kryptonian monster, by merging his own DNA with the body of Zod.
Diana Prince/Wonder Woman, who's seen the carnage unfolding, returns to help. Batman lures Doomsday to the docks, where he left the Kryptonite spear he intended to use on Superman. The three of them team up to take down Doomsday, Superman taking off briefly to rescue Lois who's become trapped while trying to retrieve the spear. Realising he has to use the spear to kill Doomsday which will also fatally weaken him, he tells Lois he loves her before flying off, spear in hand.
He pierces Doomsday's chest, but is himself stabbed, and they both die. Batman and Wonder Woman are at his side while Lois cradles his body, but nothing can be done. Superman receives a funeral with full honours, while Clark is buried in Smallville. His mother gives Lois the engagement ring Clark was intending to surprise her with, and she puts it on.
Bruce and Diana are at Clark's grave - he tells her that they need to find "others like you", and has a feeling they'll need to fight. Lex Luthor is seen in his cell having been menaced by Batman, who says "he's coming", most likely Darkseid, judging by Batman's dream/vision of Parademons. The final shot lingers on Clark's coffin, and the soil Lois threw onto it starts to float...