Coltan, the ore refined into tantalum capacitors that sit on every mobile phone circuit board, comes largely from the Democratic Republic of Congo, which supplies an estimated 18 percent of the world's supply. Reporter Juliana Ruhfus crosses into the country from Uganda at the Kasindi border, tracing the mineral from mine to market through territory controlled by armed groups, including the Mayi Mayi militia. She talks to miners working the pits by hand, to traders moving coltan across militia checkpoints, and to fighters who tax or steal the ore to fund their war effort. A UN report cited in the film has already declared the trade illegal, since the shipments bypass Congo's recognized government entirely and help finance the forces undermining it. Ruhfus plays it straight, following the supply chain rather than narrating over it, so the connection between a ringing phone and a checkpoint manned by a warlord's soldiers is left to speak for itself. The film closes on that unresolved link between consumer electronics and a war that keeps being paid for in coltan.