Sally Potter’s “The Party” starts with a close-up of a front door with a majestic lion-faced knocker. Then the door opens, and a disheveled Kristin Scott Thomas is standing there, pointing a pistol directly at the camera.

Clearly, this will be a party to remember.

But who is the intended victim? The dinner party depicted in Potter’s delightfully acidic little black comedy is, shall we say, a target-rich environment. The seven characters in the film are self-important members of London’s upper crust — politicians and intellectuals and financiers — most harboring dark and dirty secrets. They have a lot of comeuppances coming to them.

The appearance of the gun is actually a flash-forward to the chaotic end of the film. The evening starts off rather civilly, with Janet (Thomas) getting ready to receive her guests at a dinner party celebrating her recent political triumph. Taking phone calls from well-wishers, she can barely conceal her cat-got-the-canary smile and fake humility. Meanwhile, Janet’s husband Bill (Timothy Spall) is doddering in the living room with the record player, something clearly on his mind.

The guests arrive — Janet’s best friend April (Patricia Clarkson) and her elderly hippie husband Gottfried (Bruno Ganz), famed intellectual Martha (Cherry Jones) and her partner Jinny (Emily Mortimer), and hotshot financier Tom (Cillian Murphy), whose wife is mysteriously running late. That Tom immediately heads to the loo to snort cocaine and nervously finger a handgun is an early indication that this party will not go as Janet hopes.

And that’s really all there is to “The Party” — seven characters, one location and 70 minutes for them to all turn on each other with malicious glee. Potter’s screenplay releases one bombshell revelation after another into the group like hand grenades rolling across the carpet. The film abounds with darkly witty one-liners, especially from Clarkson, who tells Martha, “You’re a first-rate lesbian and a second-rate thinker.”

Maybe even funnier is Ganz as the pacifist Godfried, who tries in vain to smooth things over between the guests as tensions mount. But the film really belongs to Thomas, who finds both tragic and comic notes in Janet as she watches her entire life become unglued.

Potter shot “The Party” in lustrous black-and-white with claustrophobic staging, the characters crowding the frame, the camera often looking up at them as they argue. Part of me wished that “The Party” could have gone on longer than its brisk 70-minute running time, but Potter’s instincts to keep things brisk and nasty are solid. This is a get-together that doesn’t overstay its welcome.