I'm no stranger to behaving badly at parties. Over the years I've had my fair share of inhaling tequila shots and sliding down staircases on my face. On one occasion I provided my intoxicated chum with a plateful of cat biscuits, which he then happily scarfed - even after I revealed what they were. But even I tend to draw the line at murdering the host.

The latest demo for Dishonored - and our very first hands-on with the game - finds you infiltrating a flashy shindig at the home of Lady Boyle, the next entry on your homicidal hitlist. It's time for you to lance the Boyle, but there's a catch: there are three women here who use the handle "Lady Boyle", and you're not clear which is the one who needs to die. It could be the matriarch of the family, or either of her two daughters. And as Facebook appears to be down, your only option is to snoop about and do some detective work.

(For the record I was half-hoping that SuBo would turn out to be our target, but sadly this isn't the case.)

Right from the start of the demo, it's clear that we're playing a game with whopping ambitions. We enter the level by boat, a moody opening that quickly segues onto your first problem: how best to infiltrate the party. After a few unsuccessful attempts at freezing time and sneaking past the guards, I end up possessing one of the watchmen and walking him up to a convenient spot near the front gate. As he regains control of his body, he has just enough time to vomit his guts up (possession does this, it would seem) before I ventilate his neck. After this, I use the Blink power to teleport over the fence. Shortly thereafter I manage to find myself an invitation, and stroll in the front door.

Once inside, the demo really begins to take off. It might be possible to solve the multiple-Boyle problem by killing everyone in the house, but to do so would be to spoil a wonderful slice of level design. Boyle's manor is packed with environmental quirks that demand to be inspected. Over here, an ornate wooden toilet with a flip-down lid; over there, a papier-mâché whale that ejaculates confetti from its blow hole.

But it's the NPCs who really bring the party to life. They bitch, gossip and chatter, providing endless opportunity for eaves-dropping. There's also a particularly wry gag whereby people compliment you on your edgy choice of costume. "Oh look! There's the man from the Wanted posters!" they cry, unaware of the murderous toys that lurk inside your cloak.

Uncovering the identity of Lady Boyle requires a combination of careful inquiries and the odd smattering of miniature questing, plying the attendees with drink so as to loosen their lips. I don't want to spoil things with too many specifics, but there's also a way to complete your assignment without killing the target - an option present with such unexpected sentimentality, I found that I had to follow it through.

That's not to say that I avoided bloodshed entirely. After pursuing one diversion I found myself challenged to a pistol duel in the garden, a situation I seized upon with glee. While my opponent got all serious and chest-puffy, I quietly assumed my back-turned position and then waited. You see, the ability to freeze time tends to unbalance a good old-fashioned pistol duel. In this case, I waited for the countdown, paused proceedings, then strolled over and stabbed him in the neck - a trick that caused the onlookers to fearfully gibber about Black Magic.

Next time, I'll just walk him in front of his own bullet. Or perhaps I'll smother him with a tidal wave of rats, I'm not sure.

This kind of creative freedom is all too rare in mainstream gaming, and it's telling indeed that I spent much of my demo time saving and reloading so that I could try things with a different approach. But as sublime as it is to force-push guards into electric fences, or to dice them with razor-wire landmines, it's the design of the world that really sells me to Dishonored. Head downstairs in the Boyle house and you'll find weary servants toiling over disgusting-looking meats and glowing bottles. Peek in the pantry, and you'll spy emaciated rats scurrying about in the gloom.

For all its sinister fantasy, the world of Dishonored is remarkably believable. And on the strength of this showing, Arkane Studios and Bethesda have an instant classic on their hands.

Dishonored is due for release on Xbox 360, PS3 and PC on October 12.