The Scorpion has left on its voyage to North America. After twenty-five straight days submerged to avoid the radiation, Dwight is worried about the mental health of his crew. To relieve tension, he lets them use the periscope to look at the western coast of America, the home country of most of the crew.
One of the primary objectives of the mission is to investigate the mysterious radio signals coming from the Seattle area. The crew hypothesizes that the radio transmitter is running off of hydroelectric power—common in the Pacific Northwest—but they do not know who or what is sending the incoherent messages. They soon discover the signals are coming from Santa Maria Island, just as Lieutenant Sunderstrom predicted. Dwight will send Sunderstrom the next day to the island to investigate. Before Dwight sleeps that night, he looks at the bracelet for his wife and places the fishing rod for his son by his side.
In the morning, the submarine surfaces near Edmonds, a small town close to Seattle. Even though the radiation level is high enough to kill all life, everything in the town looks normal. The crew uses a loudspeaker to see if anyone is left alive onshore, but they get no response. The sub's radar operator, Yeoman Swain is from Edmonds; Dwight invites the young man to look at his hometown through the periscope. When nobody is looking, Swain escapes through a hatch, jumps ship, and swims to shore. Dwight tries to hail Swain, but he refuses to return. Swain will only have a few days left to live, but it will be too dangerous to take him back onboard if he stays in the radiated area for more than a few hours.
The submarine moves on to Santa Maria Island. Lieutenant Sunderstrom puts on a protective suit and takes a small boat to shore. He has less than two hours to discover the transmitter and its electrical source. On shore, he discovers a decomposing body. Sunderstrom finds the converter that provides electricity to the transmitter, and he then finds the transmitter itself. It turns out that a broken window frame has fallen out of the window and is resting in an unstable position on the transmitting key. When the wind blows, the window frame rocks on the transmitting key and sends a message.
Sunderstrom moves the window frame away the keys, and then transmits a message in Morse code saying that the submarine is safe, but that there is no life on land. He returns to the electric converter, admiring the machine before turning it off; he would feel bad letting it stay on until it simply wore out. On his way out, he finds a stack of issues of the Saturday Evening Post and decides to take the magazines with him.
As Sunderstrom is walking back to shore, he comes across a group of people sitting in lawn chairs, having drinks on a verandah facing the water. He begins to approach, but then realizes in horror that the people are all dead, and have likely been so for about a year. The submarine turns back and comes across Yeoman Swain fishing from a motorboat. He reports that he has found his family and girlfriend dead, but that he is happy because he is home and because he has his car and motorboat.