In this true story Dr. Mudd treats the broken leg of Lincoln's assassin and is sentenced to life imprisonment; he fails to escape but treats yellow fever victims.
On April 9, 1865 people celebrate the Union victory, and President Lincoln (Frank McGlynn Sr.) asks the band to play "Dixie." Five days later Lincoln is shot by John Wilkes Booth (Francis McDonald), who escapes. Booth is treated for a broken leg by Dr. Samuel A. Mudd (Warner Baxter). Mudd gives Booth a prescription and is left with $50. Mudd's father-in-law Col. Dyer (Claude Gillingwater) says states had a constitutional right to secede. Soldiers come and find Booth's boot. Mudd has his black workers chase away a carpet-bagger; but Mudd is arrested for conspiracy. After Booth is killed, eight hooded suspects are tried for conspiracy by a court martial instructed by Assistant Secretary of War Erickson (Arthur Byron) to save their country from hysteria by convicting them. General Ewing (Douglas Wood) defends Mudd, but his wife Peggy (Gloria Stuart) is not allowed in the closed trial. Witnesses testify that Mudd was a slaver, a Confederate soldier, and treated Booth. Mudd argues he was not in the conspiracy but merely a physician. Peggy visits Mudd after the guilty verdict; she sees others hanged, but Mudd is sentenced to life imprisonment on an island off Florida.
Sergeant Rankin (John Carradine) tells Mudd he will wish he was hanged and knocks him down; he shows them the moat with hungry sharks. Mudd meets Dr. MacIntyre (O. P. Heggie), who has no sympathy for him. Mudd's former slave Buck (Ernest Whitman) tells Mudd that his wife sent him. Mudd writes to her, and she works to free him. A lawyer says a writ of habeus corpus could work if Mudd could get to Key West. Col. Dyer wants to attack, and General Ewing withdraws. Buck brings Mudd a letter that they are in Key West. Sergeant Rankin searches Mudd's cell and questions Buck, then finds Mudd's cell empty. Rankin orders Buck arrested and Mudd killed. Mudd climbs a wall, and Buck is arrested. Rankin shoots Mudd; but he swims to the ocean and the boat with Peggy and Col. Dyer. Rankin boards the boat and takes Mudd back, putting him in the hole with Buck.
Dr. MacIntyre futilely demands supplies from a ship for yellow fever, and he falls ill. The Commandant (Harry Carey) goes to Dr. Mudd for help. Mudd warns the black soldiers mutinying that they will hang if they don't help him. Mudd treats the ill, including Rankin, for five days; but he needs supplies. Mudd is sick too. During a storm they signal the ship and fire cannon at it until it comes in. The Commandant sends to the President a request for clemency, and Rankin signs it first. In the final scene Mudd and Buck come home to their families.
This unjust imprisonment is well dramatized and made understandable in its historical context, showing that true stories are often the most powerful because of their reality.