If tonight's episode of Smallville happens to be your first, here's the bad news: You've missed eight years' worth of backstory. Now, here's the good news: We're about to tell you all you need to know anyway.
What's It About?
For a series that started with the simple idea of "It's Superman as a kid," Smallville has somewhat lost its way in the eight seasons so far (in part, to be fair, because you can't really do "as a kid" eight years down the line). These days, Smallville is essentially "Superman before he finally becomes Superman": Clark Kent is already a reporter for the Daily Planet (alongside Lois Lane), commuting via superspeed from his home in Smallville, and he's already saving people on a regular basis with the help of fellow superheroes like Green Arrow and the Flash. He just hasn't put on the tights yet.
Also, this Clark Kent? Kind of a clueless dick.
So Who're The Bad Guys These Days?
This season, we're finally seeing General Zod in the flesh after years of him being a disembodied voice causing mischief from beyond. Copping an idea of two from DC's current "New Krypton" comics, Zod has arrived on Earth with an army of Kryptonians; unlike the comic, though, this is a time-traveling, younger Zod who isn't even a general yet. Don't worry; he's still a fan of people kneeling before him.
Clark and friends have good reason to be mad at him nonetheless. At the end of the eighth season, Jimmy Olsen was murdered by Davis Bloome, the clone son of Zod created to take over the world, leading to Clark deciding that being human and having friends is a bad thing (Trust us, Clark, we've thought the same thing often, especially while watching this show). As if that isn't enough, Zod has appeared before, escaping from the Phantom Zone and possessing Lex Luthor for a bit.
If you're wondering where Lex is, the answer is "supposedly dead," having apparently been blown up in a plane by Green Arrow last season. Considering that he's Lex Luthor - somewhat insane, a bit of a genius and, in Smallville continuity, completely aware that Clark is from Krypton and that kryptonite is a bad thing - we're betting that'll last until producers can lure actor Michael Rosenbaum back. In his absence, Luthorcorp was placed in the care of Tess Mercer, one of the show's two attempts at moral ambiguity (Green Arrow is the other; we'll get to him in a minute). Tess knows that Clark is an alien and has superpowers (thanks to the journal of Lex's dead dad), and is responsible for bringing Zod and his army to Earth, but still... she's not all bad. Maybe.
Why Do You Keep Talking About Green Arrow?
Because the show does. Since he was introduced in season six, billionaire Oliver Queen has slowly become more and more important to the show, becoming the Batman that the producers are allowed to use. Less unwilling to get his hands dirty to get the job done (See: Potentially killing Lex Luthor) and less embarrassed to dress up in a superhero costume and give himself a dumb name (See: Being Green Arrow in the first place), Queen and his love of archery became a series regular last year, meaning he has a life beyond the show's Justice League team (which consists of Arrow, Black Canary, the Flash and a version of Teen Titans' cyborg) - which is good, considering Clark fell out with them at the end of last season (See: Clark being a bit of a dick).
They're not the only familar faces from the DC Universe in the show, however; Metropolis police detective John Jones is actually a powerless Martian Manhunter, and last year saw the introduction of the Legion of Super-Heroes, who're rumored to return this year. Also coming this year: the Justice Society of America.
What About All The Regular Folk?
Problem with Smallville is that it's run so long that no-one is just regular folk anymore; Lana left the series after becoming (a) psychic and (b) radioactive with kryptonite, which is apparently a side-effect of Kristin Kreuk wanting a career that doesn't involve Tom Welling, Chloe - still there after eight years, and the only character besides Clark to hold that honor - turned out to have healing powers and got possessed by Brainiac, only to end up taking the codename Watchtower and acting as the Justice League (and Clark)'s unofficial official IT person, and Lois... well, she's still just Lois, really: Hard-headed investigative reporter for the Daily Planet who's fallen for Clark but somehow is still unaware that he has super-powers, demonstrating that she's not a very good investigative reporter.
For those keeping track of Clark's parents: Pa still dead, Ma still in politics, which for all intents and purposes in this show means the same thing.
So Where Are We With The Story?
Davis Bloome - AKA Doomsday, AKA a cloned Kryptonian monster created by Zod and his wife - has been defeated after trying to kill Clark and succeeding in killing Jimmy Olsen, who was Chloe's (estranged) husband. As a result, Clark has decided that he's giving up being Clark Kent because it only endangers his friends, and will only be Kal-El, wearing black and saving the day in secret while necessary, spectacularly abandoning his friends when they need him, what with that whole "mourning" thing and all. Meanwhile, Tess Mercer has accidentally brought Zod and his army to Earth after being guided by mysterious forces. Everything else? Well, you'll have to watch Smallville tonight on the CW at 8pm to find out.