Although they are frustrated to see Communist enemy agents plead the Fifth Amendment and walk free, Jim McLain and Mal Baxter, special investigators for the House Un-American Activities Committee, undertake a new mission to investigate a Communist cell headquartered in Honolulu. At the airport, Phil Briggs, a Honolulu newspaper reporter, agrees to cooperate with Jim and Mal in return for a scoop. To begin their investigation, Jim and Mal present subpoenas to known Communist party members in Hawaii, but are unable to find former party treasurer Willie Nomaka. Posing as a creditor, Jim gets Nomaka's address from Nancy Vallon, the secretary of Nomaka's psychiatrist, Dr. Gelster. Then he asks Nancy for a date. Meanwhile, Sturak, an elite party member, orders Gelster, who is also in the party, to "get rid of Nomaka," as his nervous behavior and drinking habits jeopardize their security. On weekends, Jim courts the widowed Nancy and later proposes to her, and during work hours, Jim and Mal find their investigation supported by Police Chief Dan Liu and local businessmen who want to protect Hawaii from the Communist menace. A former party member tips Jim and Mal that Nomaka may soon crack, as he has been drinking heavily in public, which is against party rules. Hoping to get information from Nomaka, Jim and Mal go to the address supplied by Nancy, but instead meet Nomaka's brazen landlady Madge, who tells them that Nomaka had a nervous breakdown and was taken to a sanitarium by Gelster. As they talk, two thugs arrive to pick up Nomaka's trunk, and Jim notes their license plate numbers. Liu has the men picked up for questioning for an unrelated incident and the contents of Nomaka's trunk are photographed. Then the men are tailed to a ritzy club, which, unknown to the police, is Sturak's center of operation. When the film is developed, Mal realizes that the contents look like insurance policies, and later the Washington office confirms that they have found evidence of insurance fraud. Jim learns that Nomaka's ex-wife is a nurse on Molokai, and he goes there to meet her. Penitent for her years as a party member, she admits that she had not heard from Nomaka in years, then recently received two incoherent messages in which Nomaka accused himself of "fratricide," although he has no siblings, and mentioned returning to the religion of his childhood. Jim next visits the Shinto temple, where Reverend Ito reveals that Nomaka has been seen praying there. Meanwhile, Liu tips Mal that Gelster's phone bills included calls to Sanford Sanitarium. Mal proceeds there with a search warrant and finds Nomaka, who is in shock from the breakdown, heavily drugged and unable to talk. A local businessman introduces Jim and Mal to Edwin White, a union leader who appears to be routing out Communists in the workplace. However, Jim and Mal coincidentally receive a call from the Lexiters, a retired, first-generation Polish-American couple, who say that their son, a Communist, is working in Honolulu using the name Edwin White. Later, Madge calls, claiming she has something of interest for Jim. After dragging him to several bars, she finally hands over an opened letter from overseas addressed to Nomaka, which reveals his involvement in the sabotage of a U.S. Navy vessel, which resulted in the death of Nomaka's childhood friend and fellow Shinto. Meanwhile, Mal is murdered while following a lead, and the autopsy reveals that he died from an injection of truth serum that aggravated his heart condition. Later, Jim tries spreading rumors that the investigation is off, but Sturak recognizes the old police trick and calls a meeting of local party leaders, unaware that his club has been wired by the police. Unhappy that Gelster killed Mal, he orders the doctor to confess his party membership and implicate seven others, hoping that the authorities will believe the ring has been dissolved. Then three "essential" members will be free to continue with plans: Whelan, a labor relations counselor, and White, a labor union leader, are to create dissent among the workers and halt production, while Mortimer, a bacteriologist, is to create an epidemic on the island. After rounding up Briggs, Jim crashes the meeting and starts a brawl, and Liu and his men arrive shortly after to arrest the party leaders. Later, Gelster and two others are charged with Mal's murder, but the others are brought to a special House Un-American Activities Committee hearing held in Honolulu. Again, Jim sees the Communists plead the Fifth Amendment and go free. Later, with Nancy at his side to comfort him, Jim watches a parade of servicemen.