John Carpenter's Tales of Science Fiction: Twitch

In 1977, astronomers detected the so-called "WOW" signal: a burst of sound from deep space, near the constellation Sagittarius, believed to be extraterrestrial in origin. The next month, the astronomers sent a reply. More than four decades later, the alien intelligence has sent an invasion force of exactly one photon--and it's the kind of microscopic threat we could have never seen coming.