Skilfully crafted and smoothly assembled by London-born film maker Lucy Walker, the documentary Countdown to Zero raises the all but forgotten question of nuclear war and leaves us petrified, in almost no doubt it will happen. It's a handy guide to the history of the bomb and how to make your own. There are more than 23,000 bombs still active on Earth. Most troubling are the "loose nukes", the warheads strewn about the former Soviet countries where plutonium factories rust silently and where, it seems, Georgian smugglers have enriched uranium stuffed inside their packs of contraband fags.

Meanwhile, former CIA counter-terrorist experts create a typically American climate of fear, demonising North Korea, Iran, Iraq and Pakistan while also brazenly telling of cold war near-misses, such as when a flock of geese on the radar put Russia on red alert. It's an oppressive yet weirdly entertaining watch and Walker lines up some powerful politicos: Tony Blair confesses that the thought of terrorists trading nuclear capabilities used to "keep him awake at night"; Mikhail Gorbachev muses regretfully over his failed disarmament talks with Ronald Reagan.

The presence of the former US defence secretary, Robert McNamara – filmed shortly before his death in 2009 – reminds us not only of Errol Morris's touchstone film The Fog of War, but of how little attitudes have changed since McNamara took office and Kennedy spoke to the UN, in 1961, of "a nuclear sword of Damocles hanging by the slenderest of threads".

As this summer's movies appear to be telling us, we won't always have the X-Men around to avert nuclear catastrophe, so someone had better do something fast.