On the eve of the United States entry into World War II, civilian defense organizations are set up in Pacific coast cities. As one city prepares for a practice blackout, mechanical engineer Robert Draper is tried for the murder of his partner, with whom he invented an anti-aircraft device. Robert is found guilty and sentenced to execution; however, the blackout occurs just as a police truck is transporting him to prison, and when the truck crashes, Robert escapes. In the park he encounters Mary, a beautiful young woman who sympathizes with his plight and tries to help him get rid of his handcuffs. Mary remains with Robert even when he checks into a hotel and calls his friend, John Ronnel, for help. Ronnel, who acted as a character witness at Robert's trial, actually framed him, and now contacts the police and informs them of Robert's location. He then contacts Marie Duval, a nightclub singer who is in on the frame-up. Although Robert and Mary escape from the police, they are then commandeered by a group of enthusiastic civilian defense volunteers, who are operating a mock-hospital in a shelter as part of the blackout procedure. A pickpocket helps Robert release the handcuffs, and Mary, who is deemed "dead," is separated from Robert when she is carried by stretcher into the street. She sneaks away to the Cascade Club, where she confronts Duval, who reveals nothing. Mary decides to wait in the club, but after she leaves Duval, Ronnel emerges from his hiding place in her dressing room and kills the singer because she can identify him as the murderer. Robert goes to the club and then heads to the anti-aircraft division with Mary after discovering the division's phone number with Duval's body. There, Robert forces a confession out of Harold Kermin, who admits that he and Ronnel caused Robert's range finder to fail its test with the army so that they could sell the device to the enemy. Kermin also informs him that the base's munitions plant is going to be bombed that night by its own personnel because he and Ronnel exchanged the fake bombs for real ones. Mary, who is a telephone operator, had her co-worker and boss listen in to the confession by phone, and after they call the police, Kermin is arrested and Robert is taken into custody by the commanding officer of the base. All bombers flying test missions over the blackout area are recalled, but when bomber K-64 does not respond, Robert signals the crew by searchlight just in time so that they do not carry out their mission. With the end of the air raid, Ronnel is arrested for two murders, and Robert, now free, kisses Mary, with whom he has fallen in love.