New York Police Sergeant John Kennedy once guarded Abraham Lincoln for 48 hours while he was campaigning for President of the United States, and came away deeply impressed by the man. Kennedy has infiltrated a cabal and discovered that an assassination attempt will be made as the president-elect makes his way by train via Baltimore to Washington, DC. His boss, Superintendent Simon G. Stroud (an uncredited Tom Powers), dismisses the threat as "hogwash", as does Caleb Jeffers (Adolphe Menjou), a militia colonel with whom Stroud is meeting. Kennedy resigns on the spot to try to foil the conspirators on his own. Having already sent a copy of his report to the Secretary of War, he telegrams Lincoln, urgently requesting a meeting in Baltimore.
On February 22, 1861, he boards the Night Flyer Express train bound for Baltimore and Washington, where Inspector Reilly (an uncredited Regis Toomey) is to give him his train ticket. However, Kennedy cannot find his friend. Without a ticket, he is forced to get off by conductor Homer Crowley (Will Geer), and there are no more tickets to be had. As the train starts pulling away, Kennedy sprints aboard anyway. Among the other passengers are Mrs. Charlotte Alsop (Florence Bates), an anti-slavery writer; Lance Beaufort (Marshall Thompson), a soldier from Georgia who plans to resign and enlist in the Confederate army; his sister Ginny (Paula Raymond); and their slave Rachel (Ruby Dee).
After much searching, Kennedy finally discovers Reilly's body on the exterior platform of a car, but the corpse slips off the train as he is reaching for it. When he returns to what should have been his berth, he finds an imposter (Leif Erickson) claiming to be him and in possession of his ticket. The conductor is summoned. Fellow passenger Jeffers vouches for Kennedy and gives him a spare ticket to share his compartment.
The imposter forces Kennedy off the train at gunpoint at the next stop, planning to kill him when the train whistle sounds. Kennedy manages to grapple with him. The commotion attracts Jeffers' attention, and the colonel shoots and kills the conspirator. When they reboard, Jeffers offers Kennedy first use of the only bed in their compartment. After Kennedy appears to be dozing, Jeffers steals the derringer he had loaned the ex-policeman and shoots him. Fortunately, Kennedy had become suspicious (as Jeffers' shot could easily have hit him) and tampered with the bullet. Jeffers confesses he is in the plot in order to protect his shares in Northern cotton mills, which would be adversely affected by war.
At the next stop, Kennedy tries to have Jeffers arrested, but Jeffers obtains confirmation by telegram from Stroud that Kennedy is no longer a police officer, and it is Kennedy who is taken into custody by Lieutenant Coulter (Richard Rober). Rachel tries to give Kennedy an urgent message, but is brushed off by Coulter. Kennedy manages to escape and get back on the train. Meanwhile, the exasperated conductor is ordered to hold the train until a special package is delivered. Passenger Mrs. Gibbons (an uncredited Katherine Warren) meets and takes aboard her ailing husband.
Kennedy runs into Rachel, who informs him that Beaufort is getting off at Baltimore, not Atlanta as he had claimed. He is taken prisoner by Beaufort and tied up in Jeffers' compartment. The plotters are disappointed, however, when they receive news that Lincoln has cancelled his speech at Baltimore, where Beaufort was to assassinate him. Jeffers gets off, but as the train is pulling away, he remembers Mrs. Gibbons; he surmises her "husband" is actually Lincoln in disguise. Running after the train, he manages to alert Beaufort. Kennedy, however, frees himself and, in the ensuing struggle, sends the would-be assassin tumbling from the speeding train. Afterward, Mrs. Gibbons tells Kennedy that she is an undercover Pinkerton agent, and that his report to the War Department was read by Allan Pinkerton, who persuaded Lincoln to cancel his speech and travel incognito on the train as the ailing Mr. Gibbons.Dick Powell as John Kennedy
Paula Raymond as Ginny Beaufort
Adolphe Menjou as Colonel Caleb Jeffers
Marshall Thompson as Lance Beaufort
Ruby Dee as Rachel
Richard Rober as Lieutenant Coulter
Leif Erickson as Stranger
Will Geer as Homer Crowley
Florence Bates as Mrs. Charlotte Alsop
According to MGM records the film earned $473,000 in the US and Canada and $147,000 elsewhere, resulting in a loss of $608,000.