The hypothesis comes from John F Kennedy: "Every man, woman and child lives under a nuclear sword of Damocles, hanging by the slenderest of threads, capable of being cut at any moment by accident or miscalculation or by madness." And gifted filmmaker Walker kicks off with images of horrific terrorist attacks all over the world, noting that if terrorists get hold of nuclear weapons they won't hesitate to use them. Especially since al-Qaeda's stated goal is to kill 4 million people, as many as they say the West has killed in the Arab world.

Walker also shows us how easy it is to make a nuclear bomb. Getting highly enriched uranium is the hardest part. And that's not actually so difficult since several countries (especially Russia) have nuclear material sitting around, and it isn't well guarded. Footage of sting operations to catch smugglers is shocking, as it shows how easy it is for a non-professional to get hold of this material, transport it and put it on the black market. Remember how easy it is to smuggle drugs into America and Europe.

The film includes a chronology of unseen accidents involving American nuclear material. These happen all the time apparently, and the material is rarely recovered. Among a list of potential Armageddons, there's an alarming account of an event in 1995 when nuclear annihilation almost took place, prevented only because Boris Yeltsin failed to follow protocol. And vox pops show just how ignorant average Americans are about this subject.

The film is skilfully shot and edited, with a forceful sense of urgency augmented by Peter Golub's ticking time-bomb score as interviewees speak with gravity and authority, causing out minds to spin with the implications of all of this. And the scariest thing is that this gloomy film almost makes us wish that, if humanity comes to this, we're caught in the first blast. Because the filmmakers' final call for a banning of all nuclear weapons seems more than a little idealistic.

Teenager JoEllen grew up without a father, fantasising about what he might be like. So she decided to find him through the sperm bank her mother Lucinda used. Before finding him, JoEllen registered to find any possible siblings. Sure enough, she found Danielle in New York, and a story about their meeting landed on the front of the New York Times, triggering reactions from dozens of siblings all over the country. It was also seen by Donor 150 himself, Jeffrey, a free-spirited California beach bum living in a motor-home with dogs and pigeons.

Now 20, JoEllen narrates the film with an introspective honesty that's disarming, taking us on this journey with her. Meanwhile, we're also introduced to Jeffrey and his extraordinary, eccentric life. Now 52, he's happy to be off the grid, doing his own thing, and stunned by all of these children he fathered. Intriguingly, aside from the physical similarities, his biological children share his interest in philosophy and love of animals. And the filmmakers are present for key meetings between Jeffrey and his long-lost progeny.

Skilfully shot and edited, the film's intimate approach adds complexity to everyone on screen. Watching Jeffrey walking along the beach helping people is fascinating, as is seeing his donor profile report, written when he was 28. His quirky but articulate commentary is fascinating, as he describes his past as a stripper-gram and the reasons he decided to donate sperm, which went beyond making enough money to pay his rent for eight years.

It's remarkable to see these people find their common ground, even though they never imagined each other's existence. And these half-siblings discover a strong bond, changing the shape of their families in a rather wonderful way. It's also intriguing to see them try to work out how Jeffrey fits into their extended network, because he's essentially the outsider. But best of all is how the filmmakers stay out of the way and let these people tell their own story.

YouTube in 192 countries users shot their videos on 24 July 2010, and filmmaker Macdonald arranges it as a day in the life. It starts at midnight as we see night owls, early risers, breakfast routines and then food gathering and preparation, travel, sundown and sleep. Along the way, the participants are asked what's in their pockets, what they love the most and what they're afraid of.

It's a fascinating project, and the submissions show a huge level of creativity, not to mention some astonishingly gifted camerawork. The clips flow naturally into each other, and part of the thrill of watching it is seeing what comes next, as well as trying to figure out where in the world we are now. A few people get extended segments, while others return to the screen repeatedly as the film progresses. Overall, the work has an interesting shape and tone, carrying us through a range of emotions.

Although this also is one of the problems: Harry Gregson-Williams' insistent musical underscore tells us what to think and feel, sometimes changing the tone of the images to match whatever we're watching (such as intercutting screaming rollercoaster riders into the gloomy montage of riots and crime). And some things are oddly missing, such as scenes of grief or death and any hint of sexuality. Surely these are two rather important aspects of human life.

There's also only one event (the Love Parade tragedy) that places this film into context on this particular day. Everything else is fairly generic, with births, engagements and marriages (including an Elvis chapel in Vegas). The most memorable segments are father-son pieces from Japan and Britain. And the only multi-cultural images are in the shots of a Korean bicyclist in Kathmandu (Okhwan Yoon has been riding around the world for 10 years). Watching all of this is exhilarating, funny, scary, romantic and sometimes very emotional. It's thoroughly involving, and yet we can't help but wish it was a little more earthy and real.

A natural-born driver, Senna was a soft-spoken, likeable young guy whose wealthy family backed his career from the start, indulging in his passion for go-karts. they also supported as he fought his way into Formula One and rapidly rose to fame due to his sheer talent. As he speaks, we see the importance of religion in his life, his deep desire to improve life for Brazil's children, and how he learned from mistakes as he earned three championship titles. He also boldly dismissed the politics of the sport and stood up to anything that was unfair.

The film carefully traces the controversies and scandals in his career, including messy clashes with fellow driver Prost at the end of the 1989 and 1990 seasons, and bull-headed official decisions that seemed to conspire against him. But he was such a beautiful driver that he became a global hero, and in his homeland there were three days of national mourning after he was killed at age 34 in a crash at the 1994 San Marino Grand Prix. This resulted in significant changes to Formula One safety, and no driver has died in competition since.

All of this is told through archive footage and off-screen narration from people who watched his career. These include his mother Neide and sister Viviane, fellow drivers, managers and journalists. But it's Senna's own voice, of course, that gives us the most insight, as we see what he thought and felt about racing, including his reaction to friends who were seriously injured in crashes. And camera footage from on-board his cars is exhilarating.

As entertaining as it is, this is a documentary created in the editing. There is no new footage, as director Kapadia collects terrific clips from a variety of sources (including behind-the-scenes meetings and home movies) and assembles them into a narrative structure that tells Senna's story chronologically, accompanied by Antonio Pinto's moody score. And since most of it is essentially TV footage, it doesn't feel hugely cinematic. But Formula One fans will find it essential viewing.