In 1780 Major John Boulton is recruited by Colonial intelligence as a counterspy who will feign desertion to the British forces. His mission is to discover the identity of an American traitor with the code name Gustavus. Although prominent Tory Dr. Odell suspects Boulton of being a double agent, the spy wins the friendship and respect of British spymaster Major John Andre and, in doing so, discovers that the traitor is none other than American hero, General Benedict Arnold, who is planning to surrender the key colonial position of West Point to the English.