As the Battle of the Atlantic rages in the spring of 1943, Victory Munitions takes on a vital role: production of newly developed sonar equipment for use against German U-boats. However, a saboteur is already on site. Covertly recruited and sworn to secrecy, Gladys must now spy on her coworkers and friends. But without the help of Lorna, Kate, Betty, and Vera, how will she find the traitor?