When an explosion aboard a top-secret test boat kills a young naval officer and severely injures Matthew Reynolds, Martha and his friends are forced to investigate. How could such a thing have happened? Murphy discovers the cause and when secret plans also go missing, sabotage is confirmed. But who and why? The investigation goes beyond the Naval Torpedo Station in Newport, RI and threatens to cause an international incident.

Meanwhile, the success of Matthew's new shop disrupts the newly wedded couple's domestic bliss, and while George Evans builds a new boat of his own, Connie Worcester returns to Newport in poverty. She and Martha end up kidnapped while snooping where they shouldn't, but show their true mettle after a high-speed torpedo boat chase and some marvelous heroics.