NOTE: This spoiler was sent in by DC who notes... "I have added a few historical clarifications here and there that the movie only refers to but doesn't make explicit."
The movie begins in 1797. A carriage carries two men through the English countryside in the pouring rain; ahead, two men are beating a horse lying on the ground. The carriage stops, and the two men come out; the younger points out tiredly that if they will stop beating the horse, it may be able to recover and get up. One begins to react violently at the interference, but the other stops him, saying that the gentleman who spoke is William Wilberforce; he's seen him speak in London.
The carriage arrives in front of a home, and an older lady comes out to greet Wilberforce (Ioan Gruffudd, "Fantastic Four", "Horatio Hornblower") and his traveling companion and friend, Henry Thornton (Nicholas Farrell, "Hamlet," "Twelfth Night," "Chariots of Fire"). Marianne Thornton (Sylvestra Le Touzel) and her husband converse quietly about Wilberforce's very haggard and ill look. At night Wilberforce keeps having nightmares about slaves; at one point Thornton comes in to wake him up for his 3am dose of laudanum...which, ironically, is supposed to help him sleep. (Wilberforce also takes the laudanum to help with the pain of his colitis.)
Cut to the next day, where Marianne Thornton is chatting with the beautiful young redhead Barbara Spooner (Romola Garai, "As You Like It," "Mary Brant," "Vanity Fair") at some health-clinic sort building. Outside, Henry Thornton is in an unusual rush to get "Wilber" inside to taste the healing waters of Bath. A puzzled Wilber follows him inside, muttering that these waters have been there for years and certainly aren't going anywhere.
Just as the two men enter, Marianne abruptly changes the subject to the abolition of the slave trade and then cheerily points out to Barbara that William Wilberforce has come in. At the same time, her husband suddenly begins extolling the healing effects of marriage--and points out Barbara to Wilber. Wilber and Barbara leave, understandably annoyed at this rather blatant attempt to set them up.
The Thorntons don't give up--they seat the two single people next to each other at a meal. Wilber and Barbara are pointedly ignoring each other and the nudging looks sent by their host and hostess. After dinner, the ever persevering Thorntons unsubtly suggest that Barbara accompany Wilber to see a beautiful ivy on the far side of the garden. The two set off, and Barbara suggests a trick to get back at the matchmakers: they will pretend to argue heatedly, knowing the Thorntons will be watching. They just need to find a topic on which they disagree. Wilber is quite game and begins to offer topics as they walk. To their surprise (and unfortunately for their plan), they find rather they agree on everything from politics to having flowers in church.
They sit down, and over the next to scenes (in the garden and then in the house) Wilber tells Barbara about the last couple years fighting to end the slave trade. The next roughly half of the movie is Wilber recalling the past.
We see him as a young man not very long after his conversion to evangelical Christianity (which was in 1784). At this point, he is very much debating whether he should go into the ministry and abandon a political career altogether. His friend William "Billy" Pitt the Younger (Benedict Cumberbatch, "Hawking") tries to convince him not to, and tells Wilber of his grand plan to become prime minister (Pitt becomes the youngest prime minister ever at age 24)--and that he wants old college friend Wilber there with him.
At one point (can't remember if this is before or after the dinner mentioned next), Wilber is playing cards with some of the members of Parliament. At one point the Duke of Clarence (Toby Jones) is trying to drive up the stakes, but he has no more chips; he offers Wilberforce an IOU, but Wilberforce says with a tiny grin that Clarence has nothing he wants. Clarence then glibly offers to throw in his slave, for whom he paid 25 guineas, and has the man brought in. The slave stands there with his head bowed as Clarence goes on about having bought this slave. Wilberforce is so disgusted he leaves the game.
Pitt, still determined to convince Wilberforce to stay in Parliament, brings a group of people with him to Wilber's house for dinner. Wilber is clearly puzzled at all these guests but graciously receives them all. He starts the dinner conversation by asking a question, but before he gets a response one of the men sitting across from him pushes his plate away and unceremoniously clears the table in front of him. Thomas Clarkson (Rufus Sewell, "The Illusionist," "Tristan and Isolde," "Legend of Zorro") then places a set of chains on the area he has just cleared and begins to explain how slaves are transported in ships. The former slave Oloudah Equiano (Youssou N'Dour), sitting next to Clarkson, explains the process and shows Wilberforce the branding mark he got when he was sold. They tell Wilber that they need someone to help them to oppose the trade. Hannah More (Georgie Glen), sitting next to Wilber, tells him very quietly that they understand he is debating between doing the work of God and staying in Parliament; "We humbly suggest that you can do both."
(This is the group known as the "Clapham Sect" or the "Clapham Saints", which a Parliament member refers to later--a few paragraphs down.)
Wilberforce then goes to speak to his sometime minister, John Newton (the incomparable Albert Finney). Newton is still feeling very much the burden of having commanded slave ships, and he encourages him to go into Parliament and fight the trade.
We see Wilberforce begin to take up this cause in Parliament (first bill in 1789) and he barely ever gets a word in before he is shouted down and heckled. He meets with his fellow friends (the ones from the dinner) about how to build up evidence against the trade and gain support. They are part way through the meeting when a member mentions that there is someone else who supports them--and in walks Lord Charles Fox (the brilliant Michael Gambon), from the opposite party. The others just stare as he comes in, talking about what sort of allies and opponents they can expect, and sits down in a chair at the front. Everyone is still staring, shocked. Fox finally asks, "Do any of you saints drink?" Clarkson recovers first: "Well, this one bloody does" and offers him his flask.
Parliament is immovable. Tarleton, speaking for his side, points out the great detriment the lack of trade will cause to the working people in the port cities; others point out that the sugar cane industry will crash without slavery. Despite the research they do and present (including Equiano writing and publishing his autobiography, which becomes something of a bestseller), nothing happens. Wilberforce at one point is accused of not caring about the regular poor commoners of England, preferring the welfare of slaves over theirs.
Not long after, during a session at Parliament in which Wilberforce argues with his main opponent, Lord Tarleton (Ciaran Hinds, "Rome", "Phantom of the Opera", "Veronica Guerin"), Wilberforce rolls out a petition signed by a couple hundred thousand Englishmen calling for the abolition of the slave trade. Fox publicly comes over as the last to sign the petition, causing shock waves throughout Parliament. Tarleton tries to stall, and Wilberforce shouts out that he cannot silence the voice of the people. Tarleton looks at Wilberforce in disbelief: "The people!"
Wilberforce and his friends try year after year to get the abolition bill passed but without success. The outbreak of the French Revolution brings them low (in 1793 France declares war on Britain). Wilber and his friends are charged with sedition in advocating the abolition of the slave trade; the argument goes that France will take over the trade and thus profit, so abolitionists are traitors. It doesn't help that Clarkson in many ways is revolutionary and impatient with the slow pace of reform and with the monarchy. Pitt withdraws reluctantly from Wilberforce.
Wilberforce is run down and depressed, as are his friends. Clarkson says he is heading off to France to see everything for himself (and when he returns, lives far from London in a small country cottage); James Stephen, the lawyer, leaves England. Wilber is taken to the Thorntons' home, and we reach present day, where Wilber is telling Barbara his story. They have been talking all night into the morning.
This morning, Marianne and Henry Thornton are havingbreakfast when a clearly rejuvenated Wilberforce pops in, telling them that he's returning in London and that he's discovered he and Barbara share a very impetuous personality and that Barbara will enlighten then more about that.
Cut to a church--where Barbara and Wilber are getting married.
At the wedding breakfast (or party) after the wedding, the Wilberforces are greeting guests when Wilber passes a room and sees Pitt sitting inside by himself, having a glass of wine. He approaches, and there is an awkward silence. Wilberforce mentions that he didn't think Pitt would come to the wedding; Pitt admits he didn't think Wilber would even invite him. The two men and Barbara have a cute exchange about Wilber's pet rabbit, and then she leaves them to talk politics. Pitt explains that now that the fears about the French Revolution have passed, the Parliament will be more open again to talk of abolition, and encourages Wilber to come back and try.
Wilber then rushes about to regather his friends for another push at Parliament. They reconvene in London, rejuvenated, but older and wiser. Drawing on his research while in Jamaica, James Stephen offers a devious political sidestep to aid them in abolishing the slave trade, a backdoor slip rather than a full frontal assault. The Stephen-inspired Order-in-Council would allow for the seizure of neutral ships trading with ports under French control. Not only would this hurt France (the Brits are fighting Napoleon now), but Stephen particularly wants its side effect: it would cut slave trade profits by nearly 80%. It would attack illegal slave ships flying the flag of a neutral country (namely, American) and severely reduce the provisions sent to the slave-holding Caribbean colonies by ships using the American flag for protection. Not only does this help put a stop to much of the slave trade, the act itself looks patriotic in its action against the French--important, considering the earlier accusations of sedition. (The downside of the act, in American eyes, was that it led to the seizure of many American ships and brought on the War of 1812.)
Wilber and Clarkson, armed with their new plan, rush over to find Pitt, who is enjoying a game of golf. He sees them approach and tells them that he will finish his game before speaking to them; they stand off to the side patiently, grinning like canary-eating cats (rather scary on Sewell). Pitt prepares to putt, but now he is too curious to concentrate and gives it up in annoyance. Wilber and Clarkson explain their plan and then ask for his help. Wilber can't introduce the bill; people will know something's up. They need Pitt to find some unforgivably boring representative to introduce the bill and thus almost ensuring its passage.
Cut to Parliament: there are very few people actually there. On one side, among others, are a calm and sedate-looking Wilber and Thornton. On the other side is Tarleton and the hopelessly dull representative Pitt has chosen to offer the bill. He drones on and on.
At that moment the door opens quietly above them and Clarkson comes in, sitting up in the gallery. Tarleton notices him and immediately becomes suspicious. He looks around; there are 20-30 on Wilberforce's side and only two on his side. Tarleton knows that something is up; Thornton and Wilber lose their practiced looks of boredom and look at Clarkson in alarm, but it's too late. Tarleton immediately jumps up and demands an adjournment (recess?), saying there are Jacobite influences on the bill being offered. Wilber and Thornton wait tensely; but the man overseeing this Parliament session is hardly impressed, pointing out that the accusation of "Jacobite" is hardly enough, considering the French Revolution is over. He demands more evidence that this bill is tainted, but Tarleton can't offer any; he still hasn't figured out exactly what's up, he only knows that Clarkson has something to do with it. The session continues.
Tarleton rushes out into the rest of the Parliamentary building to find as many of his cohorts as he can. He finds one smoking in a hallway and calls for him to get back into the hall; he finds a few more here and there. But as he passes through room after room, all he sees are empty chairs and empty desks. When he finally enters the room where the MPs relax--the same room where Clarence offered Wilberforce his slave in a poker game--he finds no one. Puzzled and frustrated, Tarleton wonders aloud where everyone is; Fox appears then and replies with a straight face that everyone went to the horse races with their free tickets--a gift from William Wilberforce.
Cut to a nighttime scene in a drawing room, where Wilberforce is thrashing about (in pain from his colitis?) A very pregnant Barbara rushes down the stairs, telling him that he is scaring his servants. She goes immediately for his laudanum, but Wilber tells her that he poured it all out; he hates that it has dulled his mind so much he can barely rejoice in the fact that their bill has passed earlier that day.
Wilber later gets summoned to Pitt's home; Pitt is dying. Wilber goes in to see his old friend, and a very ill Pitt (liver disease) tells him what will happen when he dies; it seems that Pitt has arranged things politically to be to Wilberforce's advantage. He promises Wilber that the next time he makes a push for the complete abolition of the trade, he will "be pushing at an open door". Wilberforce takes his hand in his, holding onto his friend tightly; Pitt says quietly that he wishes he had Wilberforce's faith.
When Parliament votes next, the abolition of the slave trade passes to cheers and applause; Wilberforce looks to be on the verge of tears. Fox rises and gives Wilberforce a heartfelt tribute.
We are told that, when he died, Wilberforce was buried next to William Pitt.