If there's one image that unexpectedly lingers in Pierre Morel's socially conscious yet gleefully violent film, it's Sean Penn's gym-toned torso.

On several occasions, the camera lingers on the Oscar-winning actor's muscular frame: bent double over a sink staring melancholically into a steamy mirror; glistening with water under a shower; contorting on a surfboard to ride crashing waves.

We're used to seeing Penn deliver incendiary performances in heavyweight dramas such 21 Grams, Mystic River and Milk, so to see him bulking up and following the lead of Liam Neeson, who worked with director Morel on Taken, is a shock.

On the surface, The Gunman seems to appeal to Penn's outspoken political and social views: Pete Travis' script lambasts western involvement in hostilities in the Democratic Republic of Congo, which is a rich source of gold and precious minerals.

However, every time the film threatens to prick consciences, the narrative puts away its moral compass and engineers another frenetic chase or running gun battle, including some bruising hand-to-hand combat that proves Penn has been training heavily with a fight choreographer when he isn't pumping iron.

He plays mercenary for hire Jim Terrier, who operates in the Congo, where he fulfils secret contracts that would undoubtedly horrify his do-gooder surgeon girlfriend Annie (Jasmine Trinca).

Jim's colleague Felix (Javier Bardem), who manages these covert operations, accepts a contract to assassinate the Minister For Mining, who has just caused a political firestorm by announcing that he intends to renegotiate all mining contracts.

'Once the hit occurs, the shooter - and the shooter alone - is in the wind,' explains fellow mercenary Cox (Mark Rylance).

Inevitably, Jim pulls the trigger and he leaves under the cloak of darkness.

Several years later, Jim returns to Africa to atone for his sins by assisting a humanitarian relief agency.

He is attacked by machete-wielding assailants and fears the ghosts of his old life have returned to haunt him.

So Jim flies to London to warn Cox and gather intelligence from old pal Stanley (Ray Winstone) before heading to Barcelona to reconnect with Felix and Annie, who are now married.

Based on the novel The Prone Gunman by Jean-Patrick Manchette, The Gunman is a slick action thriller about a man of violence, who discovers that he cannot turn his back on the past.

Penn is better than the script deserves, and Bardem and the mercurial Rylance are both wasted in two-dimensional roles, the latter delighted to swap combat gear for the sharply tailor suits of the boardroom.

'I went from killer to cashier - don't tell anyone,' he giggles.

Morel orchestrates wanton destruction with typical bombast and brio, pausing only to let his brawny leading man take his bulletproof vest and slashed shirt off.

*** (3 Stars)