The game proceeds as follows. You and up to five other people are exploring a creepy abandoned house just like in a cliche, really bad horror movie. All kinds of awful things are happening as you search high and low for cool items until at some point, one of you becomes the traitor. At that point, that person must go fulfill whatever dark desires they have, usually at the violent expense of the other people at the table, which means its up to that small band of friends to fend for themselves and stop the aliens, vampires, zombies, or whatever.
The best part about this game is that when 'The Haunt' happens (which is the event that triggers the traitor to be revealed), it can be any number of fifty different scenarios. I won't go into the mechanics of why that is, but essentially because of how many possible plays of the game there are, you will never experience all of them. You may experience a scenario a second time after playing the game a dozen times or so, but even then the people will be different, the house layout will be different, and all of the preceding events will be different. So this game has an immense about of replay-ability.
This game has an unprecedented ability to make stories and nights you'll remember. I could reminisce about the story of how my brother once punched the head off a screaming child, but instead I'm pretty sure that story doesn't need any context to sound hilarious.
This game has a maximum of six players, and in my experience the game works best with that many people. In fact, it's a lot of fun when you have two or three experienced players and have the rest of the game played by new people. It's a lot of fun when half the players don't know what they're doing.
Betrayal is one of those games that I think everybody should play at least once. It's a load of fun and, depending on who you're playing with, can give you tales to talk about in the future. You'll never play the same game twice, and if you do play it often enough to find yourself in a haunt you've already played, you can add a house rule and pretend that the haunt roll failed, skipping it and waiting for the next omen card to be revealed.
It's also worth noting that this game is going to have an expansion released in October: Widow's Walk. I admit that you will start finding the same event and item cards over and over, even if the game you play it in is different, but this expansion adds a fourth floor to the house (the roof), and the cards it introduces brings in fifty new haunts. So basically even if you managed to get bored of this game and started to play other board games, when this expansion comes out we'll all be able to dust it off and crack it open and experience it all anew.
Oh, and by the way, it's just as much fun as it sounds to become the traitor and tear them limb from limb with your zombie hoard, or abduct them one by one with the help of your alien overlords. You may not win, but in this game, the fun lies not in challenge, but in the experience. Of course, the same can be said for everything we play, but with Betrayal it almost doesn't matter who wins. A lot of the scenarios aren't even very balanced to make the chance of victory equal on both sides, but when your friend/traitor the giant murder-werewolf is mauling you, you won't remember the fact that it killed you in the end, you'll remember that one time you managed to roll perfectly and uppercut it in the jaw against all odds. That's why this game is so good.