Movies set in modern-day Africa tend to be depressing. This is no exception. Leonardo DiCaprio plays Danny Archer, a South African veteran of the war in Angola, nowadays working as a diamond smuggler between Sierra Leone and Liberia. Sierra Leone is in the throes of struggle between revolutionary thugs and government thugs; the war is financed by the diamond smuggling.
In one of those movie coincidences, a native fisherman (played by Djimon Hounsou, the only unambiguously good main character in this) both (a) has his child conscripted by the revolutionaries, and (b) discovers a diamond as big as the Ritz, which he buries for safekeeping, and (c) lets this secret slip to Archer while they're in the slammer, and (d) Archer runs into a beautiful journalist lady, played by Jennifer Connelly.
So they all, working at cross-purposes, wind up on a quest to get the diamond and the kid. Leo punctuates everthing he says with "huh" and "right" in an (as far as I know, good) Afrikaaner accent. Jennifer seems to be channelling Tea Leoni. Arnold Vosloo, who was Marwan in 24 a couple years back, and the Mummy, in The Mummy, shows up (unsurprisingly) as a real bad guy.
There are a lot of scences of brutal savagery and carnage, all attached to a morality tale of how the diamond trade is to blame for it all. The movie's honest enough to make this simplistic and convenient premise not too believable.