The summary reads "When his mentor (Robert De Niro) is taken captive, a retired member of Britain's Elite Special Air Service (Jason Statham) is forced into action. His mission: kill three assassins dispatched by their cunning leader (Clive Owen). (imdb)"
I don't know who wrote the summary for imdb, but they clearly watched a totally different film. Statham and DeNiro are the professional assassins, and they are targeting ex-SAS soldiers who committed murders during the war in Oman, on behalf of a rich sheik. Clive Owen is part of a secret society of ex-SAS members trying to protect the targets.
I'm not sure how to succinctly rewrite that into a coherent plot summary, but it ought to be corrected.