A neutrino: one of the most intriguing elementary particles in physics. So small that they pass through the entire earth in a fraction of a second without anything getting in their way, (almost) without mass and extremely difficult to detect. But perhaps the key to answering life's big questions. What happened immediately after the Big Bang? How did we - and everything we see in the universe - come into existence? This science documentary follows international efforts to fathom neutrinos - the tiniest of everything we know - with perhaps the largest experiment ever.