A yellow-green gem set into one of Tutankhamun's necklaces sends Italian mineralogist Vincenzo de Michele looking for an explanation in 1996, after he notices the stone in the Egyptian Museum in Cairo and has it tested. It turns out to be glass, but glass older than Egyptian civilization itself, and he traces its source with geologist Aly Barakat to scattered chunks lying in the open sand of a remote stretch of the Sahara. Austrian astrochemist Christian Koeberl determines the glass formed at temperatures so extreme that only a meteorite impact could explain it, yet no crater turns up anywhere nearby, even in satellite images. Geophysicist John Wasson offers an alternative drawn from a very different desert: Siberia. He points to the 1908 Tunguska event, where an aerial explosion flattened 80 million trees without leaving an impact site, and proposes that a similar blast over Egypt could have scorched the sand into glass from the sky rather than the ground. The film follows the scientists testing that theory against the evidence.