An American officer goes undercover to unmask a Revolutionary War traitor.

In 1780, the British are gaining the upper hand in the American war of independence. The colonists' strongest defense is the fort at West Point, which is under command of General Benedict Arnold. One day, American intelligence officer Major John Bolton meets with his commanding officer, General Robert Howe, and produces a coded message removed from the body of a spy he shot the night before. The message, from someone named "Gustavus," contains top secret military information, and John concludes that Gustavus must be a highly placed person in the colonists' army. John then goes to a tavern and is searching the room of guest Sally Cameron, who is traveling to New York under a flag of truce, when she enters the room and shows him her signed military pass. John later notices a stranger entering the inn with a package for a man named Moody. Col. Winfield takes the package and says he will deliver it to Moody the following evening. John follows him to his room and takes the package at gunpoint, exposing Winfield as a British spy. During the ensuing scuffle, John shoots Winfield. After being arrested for shooting a superior officer, John is taken to Howe, who says that Winfield was a spy named Moody and shows him a letter from Gustavus found in the package. Howe proposes that John go undercover to try to learn Gustavus' identity, by escaping from prison and deserting to the British. Although it places him at great risk, John accepts the assignment. John goes to New York and calls on Dr. Jonathan Odell, in whose care the messages to Gustavus were sent, and inquires about James Osborne, to whom Gustavus' letter was addressed. Odell denies knowing Osborne, but demands at knifepoint that John leave the letter. Their struggle is interrupted by a British major, John Andre, who is about to arrest John until he mentions Moody. Odell remains suspicious, but the idealistic, gentlemanly Andre likes John and accepts his offer to spy for the British. That evening, John attends a gathering at Andre's and encounters Sally, who is Andre's lover. John is given his first assignment, accompanying civilian spies Brown and Durkin while they sabotage a chain bridge. John kills the spies, then returns to New York and reports that they were intercepted by a guard boat. After John produces a false message that he claims to have gotten from an American courier, Andre concludes that Durkin was able to deliver his letter, and, at the Cameron family's country estate, the British prepare an attack. John gets Sally alone and gently suggests that she might have some sympathy with the rebel cause. Andre instructs John to go to New York to hand deliver a message to British military leader Sir Henry Clinton, but Sally privately warns him that the assignment is a test, and he will be stopped and searched, on Odell's orders. After burning all incriminating evidence, John kisses Sally, but although she admits to having feelings for him, she believes their relationship--and the revolution--have no future. When John leaves the Cameron home, he is intercepted by British soldiers, taken to Odell and searched. The search uncovers nothing but the message to Clinton, and John is released. The following morning, Andre asks Sally to marry him, but she demurs. Word then arrives of a rebel ambush, and Sally is stunned to hear that John has been arrested and is to be executed. Although jealous over Sally's obvious interest in John, Andre nonetheless vouches for his character and secures his release. That evening, Andre asks John to attend a meeting aboard a ship between Osborne and Gustavus. At the appointed time, however, Gustavus sends a man named Joshua Smith to request that the meeting be held elsewhere, and Andre sets off alone for his "rendezvous with history." John uses a lantern to signal the ship's location to the rebels, and knocks Odell out when he grows suspicious. The British detect his activity, however, and John is forced to dive into the river to escape as the rebels open fire on the ship. The following morning, John takes rebel troops to Smith's house, and finds Andre's uniform jacket in the closet. When John learns from the rebel leader that Osborne was arrested but released into Arnold's custody, he deduces that the general is a traitor. On the road, John encounters the captured Andre and discovers that he is actually Osborne. Arnold flees to the British, and Andre is found guilty of spying by a military court and sentenced to death. John tells the court that Andre entered their lines as a soldier and should be treated as a prisoner of war rather than a spy, noting that Andre removed his uniform only at Arnold's instruction. The court is not swayed, however. That evening, Andre asks John to look after Sally. John is unwilling to abandon his friend's cause, and the next morning, asks Andre to endorse George Washington's proposal that Clinton exchange Arnold for Andre. Andre refuses, saying that to ask Clinton to betray his alliance with Arnold would insult his honor. With great dignity, and proudly wearing his uniform, Andre walks out to meet his death.