That cover, with the mullet-sporting Guardian and the young girl jumping through Jimmy Olsen's window? That's one of those metaphorical covers, because not only doesn't that scene occur in "Adventure Comics Special Featuring the Guardian" #1, but the Guardian inside the comic doesn't even look like that. First of all, he sports a moustache, and second, he doesn't wear the Guardian costume anymore. Frankly, there aren't enough superheroes with moustaches, so I think he should put the costume back on and go with the full Chuck Norris-as-superhero look, but that's not what this issue's about.
This issue has a #3 inside the S-shield on the cover, although it's actually a direct follow-up of the recent "Jimmy Olsen Special," which didn't have any kind of separate numbering on its cover. This issue expands upon a single, hinted-at scene from that earlier Special, and shows what happened when Olsen tracked down the Guardian and learned the secrets of Project Cadmus. The #3 comes in because what Olsen find out here must relate to the overall Johns/Robinson plot direction for the "New Krypton" arc running through the main Superman titles this season. And though I won't spoil the big revelation that comes near the end of the issue -- even though it's exactly what Olsen talks about at the end of the "Jimmy Olsen Special" too -- it spells bad news for a certain red, yellow, and blue Kryptonian hero.
The purpose of this issue, other than the revelation which I won't spoil, is to fill in some backstory on the Guardian. The Guardian was a big part of Jack Kirby's "Jimmy Olsen" run in the Bronze Age, and he became central to the Superman mythos with the John Byrne revamp of the 1980s. What we find out here is that the Guardian character has always been a clone. And not just one clone, as was previously thought, but a series of clones, all of whom die after a single year. So the guy who Jimmy Olsen tracks down here -- the guys who lives in a trailer like Budd from "Kill Bill," lamenting his glory days -- is the Guardian version 7.0 or something. Maybe higher. We don't exactly know how many Guardians have been born and died. It's the superhero version of Hank and Dean Venture. But this Guardian is possibly the last, because when he found out the secret, he burned down the cloning facility, destroying everything living in the giant test tubes. (Well, almost everything, as the images, but not the words, demonstrate.)
This is a perfectly fine comic about a secret conspiracy, but it's not much more than that, and it does little more than provide an impetus for Jimmy Olsen to rush back to Metropolis with his newfound information. It's mostly exposition and backstory, as the Guardian tells his story and the panels show what he's describing, though James Robinson and Pere Perez make it suitably interesting. One sequence in particular has Legion of Super-Hero fans in a tizzy (and I'm including myself in that category), as we see the Levitz-era Legionnaire Tellus, time-displaced, trapped inside a Cadmus test tube. We don't know why he's there, and we don't know what ends up happening to him, but just his presence in the story (nearly 1,000 years before his birth), in a story with the "Adventure Comics" label, has energized the Legion base and led to all sorts of speculation about the future of the team.
The story leads directly into next week's "Action Comics" #871, so if you're picking that up, you will probably want to get this one first, as it seems that Robinson and Johns are setting up the Guardian to be the street-level operative as the Kryptonian battle for the sky rages inevitably in the future.