Owen Bradwell is a courageous naval officer who returns to England in the 1930s. He believes that his career is over because he has become colour-blind ¿ but with Nazi Germany an increasing menace, the authorities cannot do without Bradwell, and he is assigned a special mission. A former acquaintance of Bradwell's has been trapped into betraying his country's secrets by a Nazi agent. Bradwell is sent to spy on the spy, and travels down the Thames on a surveillance trip. Things soon take an unexpected turn, and Bradwell finds himself in the company of a dead man, and a pretty young interior decorator called Sally. Will Bradwell triumph over the villains, and will he and Sally fall in love?