My biggest question so far about StarCraft II's first expansion, Heart of the Swarm, is what sort of zerg-oriented metagame Blizzard will wrap around the add-on's new set of campaign missions. In the first game's terran campaign, you walked good ol' boy Jimmy Raynor around his flagship between missions, overseeing laboratory research and developing new vehicle modifications that subtly changed the abilities of your units in subsequent levels. But the zerg swarm has no labs or garages to work with, so how are you going to provide the player any sort of meaningful between-mission progression? What's going to make this campaign different?

Now that I've played a couple of missions from Heart of the Swarm and played around with the stuff you'll be doing between those missions, I'm at least tentatively equipped to answer that question, and also fill you in on where we're at in the storyline with this expansion. But hang on: it's impossible to talk about any of this stuff without directly referencing the pivotal event at the end of Wings of Liberty. If you haven't finished that game and care at all about being surprised by the end of it, stop reading.

Still here? OK, so now that Kerrigan is a human again, how has the balance of power changed between the terrans, zerg, and protoss? Blizzard wouldn't say a word about how Heart of the Swarm actually opens, and where Raynor and Zeratul are in all this mess. But they did give us this teaser trailer to, uh, tease what's going on. Special guest appearance by Nova!

The playable demo dropped me further into the expansion's storyline, after Raynor and crew are out of the picture, at least temporarily. What is clear from the bit I got to play is that Kerrigan may look human (aside from that nasty tentacle hair she's still got going on) and she may be back in her old ghost uniform, but she's still very much in charge of the zerg swarm. Functionally, the between-mission interface in Heart of the Swarm isn't all that different from the one in Wings of Liberty, except it looks like you're inside someone's stomach instead of the bridge of a starship.

== TEASER ==

You've still got other entities hanging around that will fill you in on what's going on with the story--in this case a feminine advisor named Izsha, and Abathur, the evolution master--and you can click on stuff in the background scenery gets you some snippets of dialogue about the mission at hand. There wasn't much dialogue in the short demo I played, and some of it was said to be temporary, but I gathered that these zerg officiants were going through an awkward period of getting reacquainted with the new Kerrigan. Also, despite her recent metamorphosis, she's still perfectly eager to murder Warfield, Mengsk, and anyone else who wronged her in the past.

Back to the progression stuff. Going through Abathur into (where else?) the evolution chamber is how you'll access the persistent upgrades that will change the way your units work in subsequent missions. The sample levels I played only gave me access to the zergling, baneling, and roach, so no new units here (though Blizzard is obviously saying there will be some in the final game). But even those three most basic units have some really significant changes available as you finish missions and collect mutagen that you can use to evolve your units.

Some of these upgrades are pretty basic and should be familiar to anyone who plays StarCraft II online, like zergling speed or extra roach armor. But you can also get some crazy stuff like lings that train instantly and have a chance to spawn two broodlings when they die, or banelings whose acid slows targets after it hits. And once you've unlocked two of any unit's basic upgrades, you'll be able to evolve it into one of two final forms that adds some really wild bonuses. The zergling can become a raptor, which lets it leap toward a target much like the zealot's charge, or a swarmling, which gives you a third zergling per egg at no extra mineral cost. Banelings can become gorgelings, which will turn killed enemies into collectible resources, or splitterlings, which split into two when they explode and then split again to four. And while the roach's prowler sounds kind of boring with a simple burrowed movement ability, the leech, which gains 10 points to its maximum health with every kill, sounds like a real beast. These upgrades seem like they'll drastically change the way you use some units, and they would sure as hell be fun to play with in multiplayer, even though they'd probably break the game balance outright.

The most important unit in Heart of the Swarm, of course, is Kerrigan herself, and she'll be a fully playable unit in most (or maybe all) of the game's 20 missions. She's sort of like a hero unit from WarCraft III, in that she can't really be killed--she'll just burrow and reemerge at your base--and she gains new abilities between each mission. Currently, these upgrades are doled out automatically as you move through the story, and they play into different selectable "battle focuses" that completely change what set of abilities Kerrigan will have in the next mission. The Spec Ops set offers abilities like a terran ghost's, giving you an energy-based area-of-effect stun, and the ability to generate a psionic clone that does half damage. Spec Ops also gets you a passive boost to your max energy. The Corruption spec taps into Kerrigan's zerg side, giving you an armor bonus, letting you explode a small enemy unit into broodlings, and cast a poison cloud similar to the infestor's fungal growth. Those were all the abilities I got to see in the demo, but there are slots for two more full specs with five abilities each, and you're free to pick which of the four specs you want Kerrigan to use before every mission. Between the per-unit upgrades and all the choices as to how Kerrigan will play in each mission, it looks like you'll have a ton of different offensive options based on the choices you make.

Both of the levels in this demo showed the same sort of unique mechanics that made every mission in Wings of Liberty distinctive--and they both gave the sense that Kerrigan is in a weakened state and has a lot of work left to do to gather all the wayward zerg back to her command. Terran General Warfield is still rampaging around the zerg homeworld of Char, other feral zerg groups are running amok around the galaxy, and of course the protoss will take any chance to roast some zerg they can get. In the Char mission, Kerrigan was competing with a rebellious broodmother who was trying to build up her own swarm. The goal here, in addition to building up a base and amassing units, was to roam the map and collect 100 baneling eggs before the broodmother did, and I ended up clashing with her and pockets of her zerg repeatedly before I reached that goal. After hitting the magic egg number, I had about a hundred freaking banelings in a giant clump that I just rolled over the enemy base with. SO MANY BANELINGS.

The other mission took me to an icy planet that had a different, frosty visual style than anything I remember from the last game. Kerrigan was initially in pursuit of another rival zerg brood, but quickly found that a group of vengeful protoss had beaten her to the punch, and I ended up having to fight the protoss instead, along with a bunch of indigenous yeti-like creatures. The unique aspect of this mission was a period "flash freeze" that would sweep the map, rooting all your units in place for a few seconds. That got to be slightly annoying when I was in the middle of fighting the yetis, who were unaffected by the freeze, but by fulfilling a secondary objective of killing enough of the beasts and collecting their genetic material, my units were able to evolve an adaptation to the cold, letting me resist the flash freezes. Bummer for the numerous protoss bases dotted around the map, who had no such resistance. Every time a freeze came on, I rolled right through their defenses while their units stood there helpless.

These demo missions could only be played on normal--and just like in Wings of Liberty, normal is pretty much a joke for anyone who's spent much time playing RTS games online. If that's you, play these on hard or brutal, for your own sake.

Hey, here's a video showing clips of nearly everything I just talked about!

Heart of the Swarm isn't diverging too drastically from the Wings of Liberty formula, but that's fine by me, and it probably goes without saying that I can't wait to get my hands on the final product. As much as I'm looking forward to the campaign, though, it's the potential changes to multiplayer--balance tweaks, new units, the possible disappearance of old units--that really have me excited. But Blizzard really won't discuss any of that stuff yet. I will say that as a protoss-only player, I'm really hoping the campaign in Heart of the Swarm will serve as a good primer that will finally ease me into playing the highly mechanical, macro-heavy zerg on Battle.net. I'm so tired of being on the receiving end of mass muta harassment.

The weird thing about previewing Blizzard games is that the company's designers readily admit that anything you're seeing at the current moment is subject--if not likely--to change outright, but that's just about everything I can tell you about Heart of the Swarm as it stands right now. To hear what director Dustin Browder was willing to say this time around, here's one more video. It doesn't sound like there's much hope of seeing Heart of the Swarm on shelves this year, but I'm willing to wait long enough for Blizzard to get it right. The existing game sure isn't going anywhere in the meantime.