As the cast of the comic play Our American Cousin assembles backstage, aging actor Ned Emerson spins out a comparison of theater to war, while leading man Harry Hawk broods over a letter informing him of the death of a friend he hired to be his substitute in battle. Character villain Jack Matthews banters with John Wilkes Booth, who appears backstage to present him with a sealed letter announcing news that has not come to pass. Knowing Booth has concocted violent and subversive scenes in the past, an alarmed Matthews hides the letter in his pocket. As they arrive, groups of theatergoers give voice to their thoughts, while backstage a last-minute rehearsal erupts into a scuffle just as the company manager/leading lady Laura Keene enters to deliver a stern admonishment to the actors. At the sound of Hail to the Chief Keene walks onstage to welcome Abraham and Mary Lincoln, exhorting the audience to put war behind them and forget their cares for the evening of entertainment.
As the theater curtain rises on an English country estate, Mary Dundreary (Laura Keene) is helping her forgetful father, Lord Dundreary (Ned Emerson), to locate a misplaced letter, which turns out to be from a backwoods American cousin, Asa Dundreary (Harry Hawk), announcing his imminent visit to Dundreary Manor to settle some ancient business. Overhearing news of the visit of a presumably wealthy American, Lady Mountchessington schemes with her daughter Gussy to secure Asas hand in marriage. Arriving as if on cue, Asa confounds the pair with a coarse tail of herding possum on the frontier. Abraham Lincoln laughs heartily at the frontiersmans mannerisms, so comically resonant with his own public persona. The villainous Solicitor Coyle (Jack Matthews) informs Lord Dundreary that he now holds the deed to the family estate, and that only his daughters hand in marriage will forestall ruin. As the family assembles for dinner, Asa is smitten with Mary Dundreary and instantly detects Coyles plans. The rivals jockey for seats at a lavish dinner table as underfed soldiers from the theater audience look on with indignation.
As Mary Dundreary leads an adoring Asa on a tour of her modern dairy, Asa spins a yarn of a deathbed bequest by his step-father of sufficient money to save the Dundreary estate. Mary rushes off to tell her father of their salvation. Conversing quietly in their box, Abraham and Mary Lincoln look to their future after the presidency, while Booth, outside, rehearses for the assassination. Asa encounters the still eager Mountchessingtons, and having sacrificed his fortune to save the estate, declares himself penniless. As they escape from Asas comic scorns, a shot is heard. Booth leaps from the presidential box and wields a knife at the paralyzed Hawk before fleeing. While the actors attempt to continue the play, Laura Keene tries to calm the crowd, which instead erupts calling for mercy and justice. Within the presidential box, Mary prepares for a life of mourning while a surgeon tends hopelessly to Lincoln. Laura Keene, seeing the President lying on the bare floor, takes his head on her lap and bathes it. Matthews remembers Booths letter in his pocket, and he and Hawk set fire to it just as the police arrive to arrest them for questioning. While Laura Keene is left to wander the stage wondering the value of her life in the theater, a shadowy figure approaches to ask her when the play will resume. Recognizing Lincoln in the figure Keene exclaims Dont you really know what happened? Upon Lincolns silence, the audience chorus give a cryptic reply in the form of a recitation of the names of Civil War battlegrounds