Part of the New 52's origin arcs, Animal Man #0 attempts to explain the process of how the new Animal Man came to be. Animal Man is a pretty minor character in the DC Universe - he's not a superhero you see often, although apparently Swamp Thing is a collaborative force with him when the Green and the Red (we'll explain later) team up to battle the Rot. It's all complex, and even though this issue is labeled number 0, it still might get confusing to those who aren't used to the villains of the Rot and the craziness of plant and animal superheroes.
The Green is a force of plant life, and the Red a force of Animal life. Together, they try to battle the Rot - a terrible evil that threatens to destroy... well, just about everything I guess. Animal Man is an avatar of the Red, but in this first issue, the Rot is able to circumvent the natural order by sending evil baddy Arcane to trick and kill off the (surprisingly) gullible Animal Man. For a reader who hasn't had solid footing in the Animal Man/Swamp Thing/Green/Red/Rot saga, it's a huge surprise to see Animal Man go down so easy. And it's also strange to think that issue #0, an origin story, begins with a previous Animal Man murder.
But then the strange beings of the Green and Red try to explain a little bit about the process - that the Green and the Red select avatars as stand-ins, as superheroes who protect from the Rot. And Animal Man, as avatar of the Red, can be birthed - or made. That's just what happens to Buddy Baker, an action movie stuntman who is "captured by aliens" (it's made to look that way) and turned into Animal Man, a dude who can converse with animals and stuff. He can also fly.
There's a naivete to this Animal Man issue that makes things almost comical. The first Animal Man is a caricature of a superhero, and when we find him, he's protecting the wildlife against poachers in the sort of goodnatured way you might see Captain Planet speaking about saving the earth of Smokey the Bear warning of wildfires. And Baker as the next Animal Man is mostly seen carousing around, using his flying powers to do good deeds for people while also basking in his celebrity status. It's almost silly.
I say almost, because Rotworld is actually pretty cool, and that's thanks to Steve Pugh's artwork, which is filled with slithering things and slimy, grotesque details. If the writing is a little weak and uninteresting, the artwork elevates the story and deletes the blandness.
Being honest, this opening origin issue of Animal Man isn't entirely worth the read. A lot of the exposition here is simply not that useful; we don't need to know that the new Animal Man thought he was being abducted by aliens, or that he took to being a celebrity like a fish to water. For huge fans of the series, issue #0 might be worth looking into - but I would be surprised if anyone found it must-read material.