Marie Galante, Ketti Gallian is a telegraph messenger who loves the provincial French port she calls home. She delivers a telegram to a captain in the local cafe describing the route he is to take, and they return to his ship, the Hettie King, so he can compose a reply. The next morning, the captain berates a crewman for departing while Marie was onboard. Their business is illegal, so they drop her off at a seacoast town in Central America, where Marie learns that she must get to the Panama Canal to catch a ship to France.
In the office of the Governor of the Panama Canal Zone, General Gerald Phillips, [Arthur Byron], his aide Ellsworth, [Frank Darien], and British agent Ratcliff, [Robert Loraine], discuss Phillips's "gallery of suspicious characters," including retired Japanese General Tenoki, [Leslie Fenton], who owns a curio shop. Ratcliff anticipates an attempt on the Canal by Ryner, a master of disguises who kills his female accomplices. Enter tropical disease specialist, Dr. Crawbett, [Spencer Tracy], who promises Ratcliff a fine time at the Pacific Gardens cafe.
At the Pacific Gardens, Marie makes her first appearance, enthralling all the men. Miss Tapia, [Helen Morgan], tells Marie that she will receive a cut if she drinks with customers, and orders "a special"—orange juice and water. Eager to get home, Marie orders several drinks in quick succession, amusing Crawbett. She is thrilled to learn that Brogard, [Sig Ruman], owns the Parisian Bazaar; assuming he is French and will help her, she goes to his shop, where he is not French but tells her she can look at pictures of France and suggests she might be able to help him.
At the cabaret, Crawbett speaks to Plosser with authority, giving him orders and receiving reports about movements of other characters. Offscreen, Marie tells him what happened; he believes her.
Tenoki's clerk is murdered. Brogard tells Marie to learn about the comings and goings of the American officers so he can stock his shop accordingly.
Crawbett reveals himself as an agent when he calls the Bureau of Investigation in Washington, D.C. Tenoki brings Marie to his home. Plosser and Crawbett find her there and fear the worst. Tenoki asks her to reveal Brogard's secrets, but she knows none, so he sends her home. Crawbett confronts her with his suspicions. She walks away to church. He sees her kneeling before the Madonna and apologizes. She promises not to see these men, and goes to the French consul, who will not help.
Crawbett and Ratcliff meet with the governor. A telegram arrives identifying Marie as a stowaway, branding her a liar and a potential danger. The Hettie King arrives just then. Crawbett and Plosser interview the captain and the suspiciously large crew; when asked about the stowaway, they claim she escaped in Yucatán.
Marie refuses Brogard's "commission," and gives Crawbett the original telegram that she delivered to the captain. It is signed "Ryner." Ratcliff suspects that Tenoki is Ryner. Meanwhile, men from the Hettie King assemble at Brogard's shop to board a sightseeing tour of the locks and the powerhouse.
At the Pacific Gardens, Crawbett tells Marie that he cannot send her home yet; he needs to know more about these men. She cannot grasp what is at stake and only wants to go home.
Brogard discusses the sabotage plot with his men. Crawbett and Ratcliff find dynamite in the dredges near the powerhouse. Crawbett brings Tenoki to the dredge pit and finds Ratcliff dead and the boxes missing. At the powerhouse, Brogard, disguised as the foreman, receives them.
Tenoki turns out to be a good man, a Japanese spy seeking Ryner who threatens peace. Crawbett discovers Brogard's body, the mustache missing: the foreman at the powerhouse was the killer. With Ryner fleeing, Crawbett stalls his escape by staying with the supposed foreman, while Ryner shoots Marie.
The American fleet safely passes through the locks. In the hospital, Plosser and Tenoki bring roses to Marie, who now longs to stay with her two best friends. They are headed for Paris.
