In 1869, after Assistant United States Postmaster George Armstrong starts the "post office on wheels," delivering the mail by train, the mail is constantly stolen by a well-organized gang. In reaction, Congress gives Armstrong and Charles De Haven, the district superintendent, three months to prove that they can adequately protect the mail. Determined to maintain the program, Armstrong hires former Secret Service operative Steve Davis to work undercover. Using the name Steve Porter, Steve heads for Wyoming, where he tries to contact Edmund, another undercover postal inspector working as a telegraph dispatcher. Steve finds Edmund's body and near it, a horseshoe with a distinctive mark. From the local blacksmith, Steve ascertains that the shoes were made in the territorial prison. Having met attractive singer Mary Williams, Steve then visits the town music hall where she is performing. There, he learns that a horse wearing the distinctive shoes belongs to Haynes, the prison warden. Convinced that Haynes is the key to the crimes, Steve poses as a fugitive bank robber. Masquerading as a bounty hunter, Indian Joe, Steve's contact, then delivers him to prison. Once inside the prison, Steve contrives to meet Sam Wallace, a bank robber whose money was stolen by Haynes. Wallace, a former telegrapher, teaches Steve how to communicate using Morse code. Steve then signals Indian Joe, who helps him and Wallace break out of prison. During the escape, Wallace is wounded and, before he dies, begs Steve to visit Cavanaugh in Crystal City and obtain his share of the loot from an earlier robbery. In Crystal City, Steve is surprised to see that Mary is also there. Steve tries to contact Cavanaugh, and uses the marks made by his prison chains on his arms and legs to convince the gang that he is an ex-convict. Now that he has been accepted, Steve learns that Mary is also part of the gang. Cavanaugh gets him a job as a railroad telegrapher, and Steve is ordered to send a telegraph that will cause the next mail train to slow down near the location where the gang is waiting. He gets the information to Indian Joe, who then sends a telegraph to the marshal, not realizing that Ben, the telegrapher, is a member of the gang. Ben shoots Indian Joe and dispatches Mary to tell Cavanaugh that Steve is working undercover. Mary, however, has fallen in love with Steve and privately tells him what has transpired. Meanwhile, a severely wounded Joe escapes from the storeroom where Ben has confined him and struggles to reach the train. When Steve learns that despite Mary's efforts, Cavanaugh knows his real identity, he tries to telegraph the marshal. Cavanaugh shoots at him, but Steve manages to jump into a river and swim away. On the train, Joe successfully contacts the government men, among whom is De Haven. Unknown to Joe, De Haven is also a gang member and allows the train to stop. During the ensuing attack by the bandits, De Haven escapes from the train along with Cavanaugh, whom he then kills to keep his connection to the gang a secret. In the meantime, Steve, who has stolen a horse from one of the robbers, follows De Haven to the hideout. During the ensuing shootout, Mary is wounded and De Haven falls to his death. Later, after Mary is pardoned, she and Steve are wed and leave for a new life in San Francisco, where he plans to work as a postal supervisor.