The title of Lucy Walker’s pro-nuclear-disarmament tract Countdown to Zero has two meanings: a paranoiac’s ticking off down the last moments until the bomb goes off, and an exhortation to work for the cause until zero missiles and weapons remain. Synthesizing fear and optimism like that requires Walker to be incredibly ambitious in scope, and she offers up a history of the bomb and treaty talks, scientific explanations, a primer on how to smuggle uranium, and much, much more. Trying to touch, however briefly, on everything related to The Bomb means that, inevitably, much of it gets short shrift: SALT I and II are barely mentioned, but the Reykjavík Summit’s failure is inexplicably highlighted. Walker runs the same old archival test footage we’ve seen before and interviews the big names — Mikhail Gorbachev and Valerie Plame Wilson both make appearances — to reiterate her already-obvious p.o.v. She’s also prone to very literal-minded exposition; to show that a tennis-ball-size bomb could level a city, she just throws a tennis ball up against a black screen and has it rotate ominously. This is another well-intentioned but preaching-to-the-choir doc, and boring as well. Never trust a movie that ends with a moveon.org link.