In January 1865, President Abraham Lincoln expects the Civil War to end within a month. However, he is concerned that his 1863 Emancipation Proclamation may be discarded by the courts once the war has concluded and that the Thirteenth Amendment be defeated by the returning slave states. Lincoln feels it is imperative to pass the amendment by the end of the month, thus removing any possibility that slaves who have already been freed may be re-enslaved. The Radical Republicans fear the amendment will merely be defeated by some who wish to delay its passage; the support of the amendment by Republicans in the border states is not yet assured either, since they prioritize the issue of ending the war. Even if all of them are ultimately brought on board, the amendment will still require the support of several Democratic congressmen if it is to pass. With dozens of Democrats having just become lame ducks after losing their re-election campaigns in the fall of 1864, some of Lincoln's advisors believe that he should wait until the new Republican-heavy Congress is seated, presumably giving the amendment an easier road to passage. Lincoln, however, remains adamant about having the amendment in place and the issue of slavery settled before the war is concluded and the southern states readmitted into the Union.
Lincoln's hopes for passage of the amendment rely upon the support of Francis Preston Blair, a founder of the Republican Party whose influence can ensure that all members of the western and border state conservative Republican faction will back the amendment. With Union victory in the Civil War seeming highly likely and greatly anticipated, but not yet a fully accomplished fact, Blair is keen to end the hostilities as soon as possible. Therefore, in return for his support, Blair insists that Lincoln allow him to immediately engage the Confederate government in peace negotiations. This is a complication to Lincoln's amendment efforts since he knows that a significant portion of the support he has garnered for the amendment is from the Radical Republican faction for whom a negotiated peace that leaves slavery intact is morally unacceptable. Unable to proceed without Blair's support, however, Lincoln reluctantly authorizes Blair's mission.
In the meantime, Lincoln and Secretary of State William Seward work on the issue of securing the necessary Democratic votes for the amendment. Lincoln suggests that they concentrate on the lame duck Democrats, as they have already lost re-election and thus will feel free to vote as they please, rather than having to worry about how their vote will affect a future re-election campaign. Since those members also will soon be in need of employment and Lincoln will have many federal jobs to fill as he begins his second term, he sees this as a tool he can use to his advantage. Though Lincoln and Seward are unwilling to offer direct monetary bribes to the Democrats, they authorize agents to quietly go about contacting Democratic congressmen with offers of federal jobs in exchange for their voting in favor of the amendment.
With Confederate envoys ready to meet with Lincoln, he instructs them to be kept out of Washington, as the amendment approaches a vote on the House floor. At the moment of truth, Thaddeus Stevens decides to moderate his statements about racial equality to help the amendment's chances of passage. A rumor circulates that there are Confederate representatives in Washington ready to discuss peace, prompting both Democrats and conservative Republicans to advocate postponing the vote on the amendment. Lincoln explicitly denies that such envoys are in or will be in the city — technically a truthful statement, since he had ordered them to be kept away — and the vote proceeds, narrowly passing by a margin of two votes.
When Lincoln subsequently meets with the Confederates, he tells them that slavery cannot be restored as the North is united for ratification of the amendment, and that several of the southern states' reconstructed legislatures would also vote to ratify. On April 3, Lincoln visits the battlefield at Petersburg, Virginia, where he exchanges a few words with General Grant. Six days later, Grant receives General Lee's surrender at Appomattox Courthouse.
On April 14, Lincoln is in a meeting with members of his cabinet, discussing possible future measures to enfranchise blacks when he is reminded that the First Lady is waiting to take them to their evening at Ford's Theatre. He says "I suppose it's time to go though I'd much rather stay." As he walks off, he throws his gloves away. That night, while Tad Lincoln is viewing Aladdin and the Wonderful Lamp at Grover's Theatre, the manager suddenly stops the play and announces that the President has been shot. The next morning at the Petersen House, a physician pronounces Lincoln dead, and Secretary of War Edwin Stanton declares, "Now he belongs to the ages". The film concludes in flashback to Lincoln delivering his second inaugural address.