The Caldwell family is part of a Communist Cell in the San Francisco Bay area. The cell plans to blow up a U.S. ship sailing from Oakland to take war supplies to Vietnam. Erskine and Rhodes lead the FBI's investigation, which begins when Lester Milton is found murdered. Milton is a former Communist who renounced the party. John Caldwell, patriarch of the Caldwell family, is really Milton's brother. Milton was spotted by Communist operative Conrad Letterman, who is the mastermind behind the scheme to blow up the ship. The FBI races to apprehend Letterman, not knowing the full extent of the plot. Betty Caldwell, the 20-year-old daughter of John Caldwell, faces a crisis. She's in love with a Navy doctor who has been assigned to the ship at the last minute.