It's Snyder and Capullo's farewell to Batman and a capstone to their much-celebrated Batman run (by people who aren't me). Furthermore, Snyder had been planning this for years. In 2013, he'd been freaking out under Bat-pressure. Grant Morrison said, "The way to finally calm down is to give your version of Bruce a birth and a death."
As it happens, this book's quite good.
There's been a hero-pocalypse. Superheroes (and supervillains) were exterminated after Lex Luthor gave power to ordinary people. Unfortunately, ordinary people are selfish and stupid. Result: everything went to hell. Now, the world's a post-apocalyptic desert with the dead standing like zombies in a literal afterlife (it's underground) and lots of weird superhero relics. Green Lantern projected giant killer babies, howling Flash tornadoes, the fallen Spectre's cloak, etc.
I like Snyder's underlying idea. The big threat wasn't ultimately a supervillain, but just us. Ourselves. That's quite clever, in today's era of scary populist bullshit, with Trump, Brexit, border fences, the global community falling apart, etc. School shooters. Political dysfunction. Unfortunately, I don't think he's managed to put it in this book, because it contains no ordinary people. They're not being studied or examined. They're not here! We've no idea how they live in this post-superhero world, or indeed if they still exist. They might have been wiped out. We don't know. Instead, the enemy is a supervillain called Omega who's gained control of Darkseid's Anti-Life Equation. In practice, Snyder's core idea (i.e. blaming ordinary people) boils down to a vote. There was a referendum to which the heroes unwisely agreed. It went wrong, everything collapsed and now the world's florid superhero nonsense, but in a different flavour.
One day, I'd love to see Snyder's idea tackled intelligently. Don't expect that here.
I also don't understand Omega's plan. Is he trying to brainwash the few surviving heroes, or ordinary people? Why? What's the point? Also, why didn't Snyder use this to examine the difference between the traditional dark, brooding Batman and his more optimistic reboot? (Throughout his run, Snyder's Batman was supposedly inspiring us all to be better people, not to give up and to keep trying.) His story here could have literally brought a more Miller-esque Batman into conflict with his own version. There's a debate, yes, but it's only tangentially addressing that dichotomy in views of the character.
That said, though, the book's a pretty wild trip. The continuity-busting doesn't matter, because it's just another Elseworlds. (It borrows the "infinite Batman clones" idea from Snyder's run, but that's a franchise-killer in my opinion.) For the time being, here, it's fun to follow along with the romp in this dark, crazy future. Swamp Thing! Martin Manhunter! Lots of ideas and dead superheroes.
It also makes fantastic use of the Joker. He's a head in a jar. Batman carries him around. The Joker's head can talk... and he's funny! He gets some of the best Joker dialogue you'll see and he wants to be the next Robin. Snyder understands the subtext of these iconic superhero arch-enemy relationships, as is shown here in both Batman-Joker and in Superman-Luthor.
To be honest, I don't think this was a particularly clever or difficult story. This kind of thing almost writes itself. Apocalypse, death of familiar faces, yadda yadda. Put a pile of Elseworlds in a blender and press "go". It could almost have been generated by brainstorming a class of ten-year-olds, although they wouldn't have written the Joker and Luthor this well. Oh, and it starts with a fake-out that's theoretically clever but in practice an obvious trick opening. It works, though. It's a laugh. Also, its Jim Gordon looks like Stan Lee.