It is 1999 and Sierra Leone is ravaged by major political unrest. Rebel factions such as the Revolutionary United Front frequently terrorize the countryside, intimidating Mende locals and enslaving many to harvest diamonds, which fund their increasingly successful war effort. One such unfortunate local is fisherman Solomon Vandy (Djimon Hounsou) from Shenge, who is separated from his family and assigned to a workforce overseen by Captain Poison (David Harewood), a ruthless warlord.
One morning, Vandy discovers an enormous diamond in the river. Captain Poison tries to take the stone, but the area is suddenly raided by government troops. Vandy buries the stone before being captured. Both Vandy and Poison are incarcerated in Freetown, along with Danny Archer (Leonardo DiCaprio), a white Rhodesian gunrunner jailed while trying to smuggle diamonds into Liberia. They were intended for Rudolph van de Kaap (Marius Weyers), a corrupt South African mining executive.
Hearing of the pink diamond in prison, Archer arranges for himself and Vandy to be freed from detention. He travels to Cape Town to meet his employer: Colonel Coetzee (Arnold Vosloo), an Afrikaner formerly with the apartheid-era South African Defence Force, who now commands a private military company. Archer wants the diamond so he can sell it and leave the continent forever, but Coetzee wants it as compensation for Archer’s botched smuggling mission. Archer returns to Sierra Leone, locates Vandy, and offers to help him find his family if he will help recover the diamond.
Meanwhile, RUF insurgents escalate hostilities; Freetown falls to their advance while Vandy’s son Dia is among those rounded up to serve as a child soldier under a liberated Captain Poison. Archer and Vandy narrowly escape to Lungi, where they plan to reach Kono with an American journalist, Maddy Bowen (Jennifer Connelly); in exchange, Archer will provide her evidence of the illicit diamond trade. The trio arrive in Kono after a harrowing journey, where Coetzee and his private army—contracted by the Sierra Leone government—prepare to repulse the rebel offensive.