特立独行、技术高超的杀手杰克（乔治·克鲁尼 George Clooney饰）来到意大利乡下的一座田园小镇，打算执行完这次任务后金盆洗手——为女杀手组装一支专用步枪。在这相对简单平静的环境里，原本低调谨慎的杰克出乎意料的与当地居民建立了深厚的友谊。不仅如此，他还邂逅了美丽的妓女克莱拉（薇奥兰特·普拉西多 Violante Placido饰），一段浪漫的爱情由此展开。然而，这杀手生涯中的最后一次任务，却远没有想象中的简单。 本片根据马丁·布斯的小说《一位非常低调的绅士》改编而成。
Jack (George Clooney), an American assassin, is relaxing in a remote cabin at Dalarna, Sweden with his lover, Ingrid (Irina Björklund). The two decide to take a walk on the frozen lake nearby. While walking, Ingrid notices some footprints. Jack immediately becomes suspicious and quickly drags Ingrid to cover behind a large boulder as someone begins shooting at them. Jack kills the shooter and then tells Ingrid to return to the cabin and phone the police while he searches the body. As she rushes past him, he shoots her in the back of the head. Jack suspects there may be another shooter and goes in search of him. He comes up behind another armed man and kills him.
Jack travels to Rome, Italy where he contacts a man named Pavel (Johan Leysen). Pavel is the person who provides Jack with his assignments. They don't typically meet in person. Pavel insists that Jack cannot stay in Rome and sends him to Castelvecchio, a small town in the mountains of Abruzzo. Jack arrives in Castelvecchio and immediately feels uncomfortable, so he drives down the road to, Castel del Monte, throwing the phone Pavel had given him out of the car window on the way.
While he is hiding out in Castel del Monte, Jack is befriended by an elderly priest, Father Benedetto (Paolo Bonacelli), who senses that Jack is not who he claims to be and is struggling to find his way. The father pressures Jack to seek absolution, something which Jack, now going by the name 'Edward' and posing as a photographer, resists.
Jack contacts Pavel, who offers him another job, a job that won't require him to kill anyone. Jack eventually decides to take the job and meets with a shady Belgian, named Mathilde (Thekla Reuten), who wants him to build a compact gun with the firing capacity of a submachine gun and the accuracy of a rifle. Jack begins work on the weapon as he begins patronizing a local prostitute, named Clara (Violante Placido). At the same time, Jack realizes that he is being followed by a young Swedish man (Samuli Vauramo) who is probably associated with the people who tried to kill him at Dalarna.
One day, Jack meets with Mathilde to test the weapon he has made in the nearby forest. She asks for a few more alterations to it, as well as some special ammunition.
While having coffee one day, Clara and a friend, named Anna (Anna Foglietta), see him and come inside to say hello. Clara invites Jack on an actual date, and the two begin to deepen their relationship.
One night, as the Swedish assassin follows him, Jack manages to get himself into position for an ambush, but just when he is about to shoot the man from behind, one of his neighbors who rides a Vespa scooter, comes speeding along, alerting the assassin who fires, missing Jack but killing the man on the scooter. Jack takes the scooter and chases after the Swede, who has jumped into his car and sped off. Jack shoots at the Swede, breaking out the back window of the car, then a front tire. The car crashes. Jack has to lay down the scooter to avoid crashing into the car, then gets up and runs to the driver's window, smashing it out with his elbow. He reaches in and grabs the Swede, pulling him partway out of the window, then twisting and breaking his neck.
