In the streets of Glasgow, Bart (BOB HOSKINS) is a feared gangster who possesses a powerful tool that he uses to make sure those who owe him money pay up. It's not a big gun or other such weapon, but rather a human killing machine otherwise known as Danny (JET LI). Having adopted the martial arts expert off the street when he was just a boy, Bart has trained him much like a dog, including the use of having him wear a collar. When it's on, Danny is a peaceful and subservient man-child, but when it's taken off and Bart gives the orders, Danny unleashes a brutal fury on whoever his target might be.
On one of their latest stops, Danny is told to wait before storming in as the enforcement. While in his passive mode, he encounters Sam (MORGAN FREEMAN), a friendly, talkative and blind piano tuner whose calm and pleasant demeanor works its magic on Danny. As does the piano Sam's working on that elicits long repressed memories of his childhood and some tragic, pivotal event from it.
When Danny doesn't respond in time to Bart's signal to come in, the master isn't pleased with his "dog," although a new development - battling in fight-to-the-death matches with various brutes -- suddenly puts him back in good graces with his boss since Danny's guaranteed victories mean big bucks for him. Yet, another unexpected development sets Danny free.
He then seeks out Sam who ends up taking him back to his house where the now mousy man-child meets Sam's 18-year-old stepdaughter, Victoria (KERRY CONDON), who's going to school in Glasgow from New York. The two don't ask any questions about Danny, his previously sheltered existence or the collar around his neck. Instead, they view him as family and proceed to teach him about the outside world and its various trappings.
When Bart and his minions reenter the picture and want Danny back as their lethal killing and money-making machine, however, Danny finds himself having to fight for his newfound life and liberty.