Quite simple, really. 2 to 4 players. One player plays as the Genestealers, the rest get a Terminator squad each (or one player may play multiple squads, but it’s not as fun, since a big part of the fun of playing as the Marines is inter-squad bickering). A marine squad is composed of 5 marines : a sergeant (who gets an energy sword and a storm bolter), three regular marines (storm bolter+powerfist) and a heavy flamer.
Before the game starts, all players agree on the mission they’re going to play, from the rulebook. Each mission has its own floorplan and objectives, and dictates how many “blips” the genestealer player gets per turn. Usually missions involve crossing the board to reach a specific room in order to either set it on fire, or have a marine fiddle with a computer. Some missions are also pure survival, seek & destroys (kill X genestealers, then come back) and so forth.
The marines have 4 action points each, plus 1D6 “squad action points” per turn than can be used for any marine in the squad. Moving one square forward, shooting, turning 90° all cost 1 point. Moving backwards costs 2. The average corridor is 6 to 8 tiles long, 1 tile wide, the average room 3x3. Marines can also be ordered to go on “overwatch”, which lets them fire during the genestealer turn. The kicker : Marines can’t share a tile, or cross a tile with another marine in it, and can’t fire over the head of each other. Since the Hulk is very cramped, and marines are very, very slow, positioning is key. And the flamer is never where you want it to be :p.
Regular marines kill stealers on a 6+ on two dice, but if they roll a double, their weapon has just jammed, and they need to spend one AP to clear it. That happens a lot on overwatch, where they get to fire once each time a genestealer in their line of sight moves. Continuous fire (firing multiple times at the same target) gives the marine +1 each time.
Marines can also fight hand-to-hand, but they’re at a big disadvantage there. They roll one die and kill on 5+, while the stealers roll three dice and kill on 3+. Only the sarge gets a break, he rolls two dice and kills on 4+ IIRC.
The flamer’s special ability is that it can set tiles or rooms on fire (duh), burning everything to a crisp and blocking the way for one turn. The problem is that fuel is at a premium : he only gets 6 shots worth of woosh for the whole game. After that, he’s dead weight.
For their part, genestealers have 6 action points each, can move diagonals and can turn 90° for free. They can’t shoot at all, but as stated earlier they rip marines to shreds when they close in. Every turn the stealer player picks a number of “blips” from a bag. Those are little tokens with a number on them (from 0 to 6) that represent returns from the marines motion sensors (à la Aliens). They can only enter the board on a number of tiles, and only if the marines aren’t within 6 spaces of those tiles (controling the reinforcement spots is crucial for the marine player who doesn’t want to get sandwiched all to hell). Blips move 6 tiles per turn while they’re out of sight of the marines. When a blip is seen, it’s flipped over, revealing the number. The genestealer player then gets to put as many genestealers on the board as he can, up to either the blip’s number is reached, or there are no more free tiles around the blip’s position (that happens a lot when you stupidly move a 6 blip in a corridor). Those genestealers are “fresh” and get their full 6 APs no matter how many spaces the blip has moved before.
That’s about it for the rules. It’s that simple. Oh, yes, I forgot, there’s also the problem of doors, which cut vision, cost 1AP to open or close, or can be shot open (but then, obviously, they can’t be closed back. Not that it matters because marines like open doors).
For most of the game, the marine player(s) will move inch by inch, dreading corners (since moving close to a corner means you’re liable to get a 3 blip “pop” right in your face, with very few tiles to cover) and crossroads. The genestealer player will get to play with tens of models, who die very fast, but come back just as fast. The longer the game, the worse the marines are screwed, and the more blips pile up in the dark corners of the map. And then the rearguard’s gun jams. 