“They’re here already!” The ’50s was chocked with mutants — giant bugs and amphibians, revived dinosaurs and pterodactyls, men who grew to Colossi or incredibly shrank to nothing — but what if the monsters looked just like us? What if they were aliens, hatched from sea pods, who could imitate everything human, except emotion? And what if the only way to retain your humanity was … not to sleep? That’s the premise of Jack Finney’s novel The Body Snatchers and the Don Siegel film version, which rises slowly, surely, from suburban realism to end-of-the-world hysteria. Kevin McCarthy is the doctor who suspects an otherworldly plot, Dana Wynter his imperiled ladylove. The film has been remade three times (this year as The Invasion with Nicole Kidman), but the Siegel version is the simplest, smartest and scariest.