Batman and Wonder Woman have arrived at the Fortress of Solitude to pay their respects to Superman on his birthday. Wonder Woman seems unimpressed at Batman's gift-an envelope containing cash-but he admits that it is hard to shop for "the man who has everything". Upon arrival, they discover their Justice League teammate is standing in the middle of the room, unaware of his surroundings, and with a mysterious alien-looking organism firmly planted on his chest. Wonder Woman realizes the Fortress has been breached, and flies off to investigate.
What they do not immediately realize is that the Last Son of Krypton has been subdued by a telepathic plant from a distant galaxy that renders its victims catatonic and trapped in a dreamworld created from their heart's desires. Superman's mind is now trapped in a most tantalizing prison: a Krypton that never exploded. He lives on a small farm, married to a Kryptonian woman named Loana, with a young son, Van-El, Krypto, and Brainiac, whom he visualizes as the helpful AI attendant of his home.
In the real world, Superman is still incapacitated as Mongul appears before Batman, holding an unconscious Wonder Woman. As he boasts about taking over the planet, Wonder Woman comes around and knocks him down with a kick. As the two face each other, Batman intervenes, prompting Mongul to reveals the origins of the plant. With Superman trapped in a world he himself has created, Mongul presumes that conquering Earth will now be exceedingly simple. Batman futilely tries to buy time and gather clues about the plant, but Wonder Woman's rage cannot be contained. She engages Mongul, though it is apparent from the start that she is outmatched.
Kal-El's journey of the mind takes him before his father Jor-El, now a tired old man forever shamed by his premature warnings of Krypton's destruction. However, Superman now sees the cracks in his false reality, manifesting as increasingly frequent tremors. As he takes Van-El to the roof-mounted zeroscope, the words of his friend Batman, ringing in the real world, begin to slip through to his subconscious. As Batman pulls frantically at the Black Mercy in the real world, Kal-El, now realizing he is simply imagining his new life, tearfully says goodbye to a son who can never exist. As Krypton explodes around him, Kal-El promises Van-El that he will never forget his homeworld and the life he could have lived.
At last, the alien plant comes free, but it immediately clamps onto the chest of Batman, who now slips into his own dreamworld - he is a child again walking home from the movies with his parents, reliving the night they were murdered. This time however, Thomas Wayne retaliates and proceeds to repeatedly punch the gunman, Joe Chill, who killed him in reality.
Realizing the situation, Superman tears after Mongul, and a brutally beaten Wonder Woman crawls to Batman's side to help snap him out of his dreamworld. Nearly blind with rage, Superman batters the would-be tyrant, even using his heat vision to burn Mongul's chest. However, before he can strike a decisive blow, Superman is distracted by a statue-his parents holding Krypton aloft. In his moment of hesitation, Mongul gains the advantage.
Bruce begins to hear Diana calling out her name through his subconscious and this helps him break free of the plant's illusion. Like Superman's dream, Bruce's turns sour when the gunman regains his advantage and shoots Thomas in front of his horrified family. As the shot rings out, the Black Mercy comes free of Batman's chest.
Just before Mongul can finish Superman off, Wonder Woman throws the plant at him. It strikes Mongul, freezing him in place and putting him under its spell.
As the trio regroups, Wonder Woman gives Superman his birthday gift: a new breed of rose named after his homeworld. Superman takes it gladly, before solemnly swearing to his parents' image that he will never forget Krypton. Turning back to the comatose Mongul, they ponder just what sort of dream the plant is causing him to experience. Batman says that whatever it is he's dreaming about, it's far too good for him, and indeed, Mongul's beaten face smiles weakly as cries of agony and sounds of war echo in his mind.
TL;DR - Batman and Wonder Woman (or another group of heroes more connected to Superman, like Supergirl and Steel?) find Superman under the control of an alien (Black Mercy) after Mogul has tricked him with a present on his birthday. The team has to save Superman from this threat, whilst he experiences what life on Krypton would be like for him if it wasn’t destroyed. Once freed, Superman takes the fight to Mogul whilst Batman experiences what would happen if his Father stopped Joe Chill from killing them (This could be replaced with Supergirl experiencing Brainiac attacking Krypton).
