The film begins with Wilberforce severely ill and taking a holiday in Bath, Somerset, with his cousin, Henry Thornton. It is here that he is introduced to his future wife, Barbara Spooner. Although he at first resists, she convinces him to tell her about his life. The story flashes back 15 years to 1782, and William recounts the events that led him to where he is now. Beginning as an ambitious and popular Member of Parliament (MP), William was persuaded by his friends William Pitt, Thomas Clarkson, Hannah More and others to take on the dangerous issue of the British slave trade which led him to become highly unpopular in the House of Commons amongst the Members of Parliament representing vested interests of the trade in the cities of London, Bristol and Liverpool.
Exhausted, and frustrated that he was unable to change anything in the government, William becomes physically ill (the diagnosis in the film is colitis, most commonly known today as Crohn's disease), which brings the story back to the present day. Having virtually given up hope, William considers leaving politics forever. Barbara convinces him to keep fighting because if he does not, no one else is capable of doing so. A few days afterward, William and Barbara marry; and William, with a renewed hope for success, picks up the fight where he had previously left off, aided by Thornton, Clarkson and James Stephen. In time, after many attempts to bring legislation forward over twenty years, he is eventually responsible for a bill being passed through Parliament in 1807, which abolishes the slave trade in the British empire forever.