The House Un-American Activities Committee (HUAC) fails once again to get convictions of known American Communists, who stand behind the Fifth Amendment during their Congressional testimonies to avoid self-incrimination.

Investigators Jim McLain (Wayne) and Mal Baxter (Arness) are sent to Hawaii to track Party activities, with hopes of gaining enough hard evidence against members there to earn convictions.

They become interested in everything from insurance fraud to the sabotage of a U.S. naval vessel and plans to have local unions go on strike to prevent the loading and unloading of ships on the Honolulu docks.

After receiving useful information from reporter Phil Briggs (Vernon "Red" McQueen), the agents begin searching for Willie Nomaka, a former party treasurer, who is being treated by psychiatrist Dr. Gelster (Gayne Whitman).

Meanwhile, two of the communists kidnap Baxter, and Gelster accidentally kills him by giving him an injection of Sodium Pentothal truth serum seeking to learn how much he had discovered.