The 27th Day (1957)

Five Earthlings from different countries (including Gene Barry of "War of the Worlds" and lovely English actress Valerie French) are kidnapped by a Klaatu-like alien who gives each of them a palm-sized transparent case containing three silver capsules. The capsules have the power to make millions of humans vanish, without harming animals or causing destruction.

The alien's race desperately needs the planet Earth, but they are morally opposed to conquering it, so they give the war-pron Earthlings the ability to finish themselves off without devastating the planet in a nuclear war.

Don't expect any special effects except for two brief clips from "Earth versus the Flying Saucers", one space scene from "The Day the Earth Stood Still", and a small-scale test of the alien weapon. But the interior of the spacecraft is nicely done. This is an intelligent and uplifting movie, done on a small budget, although it's a bit too talky and actionless for some taste. John Mantley wrote both the screenplay and the original novel. In the novel the capsules had a somewhat more far-reaching (and disturbing) effect on humans than they do in the film. [Originally co-billed with "20 Million Miles to Earth"]