It is a typical morning in the Wetherly house. Carol is trying to get her three children (Brad, Mary Liz, and Scottie) up and ready for school. Her husband Tom and older son Brad go on a quick bike ride. Tom is rather critical of Brad, especially when his son is unable to reach the top of a steep hill. On the way home they greet an elderly neighbor, Henry Abhart, and Mike, the owner of a gas station. Mike has a special needs son, Hiroshi, who sometimes goes fishing with the Wetherlys.
Carol is helping to direct a play at the elementary school. Since their California town is named Hamlin, the children are putting on "The Pied Piper of Hamelin." Mary Liz provides a piano accompaniment and Scottie plays a little crippled boy.
That night, Carol is unable to sleep. She wakes Tom, worried about what to get Brad for his thirteenth birthday. Tom sleepily reminds her that it isn't for another two months. They discuss how fast Brad is growing up and Carol decides to start keeping a diary.
The next afternoon, the children are watching TV in the living room. Carol is listening to the answering machine messages. There is one from her mother in Chicago. Tom is in San Francisco on a business trip. He leaves two messages; one that he will be home for dinner and another one stating that he won't be able to make it after all. The TV picture is fuzzy and Brad fusses with the antenna. The telephone rings and Carol answers but the call is apparently lost. Suddenly the program is interrupted by a special report. Bombs are falling up and down the east coast and in other locations, including nearby San Francisco. Citizens are urged to stay off the telephones as the lines must be kept open for official use. Stay tuned for a message from the White House. Just then the TV goes off completely and loud sirens are heard. A bright light is seen outside. Carol screams for the children to get on the floor and cover their eyes.
After the sirens and light are gone, people begin venturing outside their homes. A child is heard crying. Everyone is asking what happened. Later most of the neighbors gather at the Abhart home. Carol leaves a note for Tom in case he comes home. Henry is a ham radio operator and has been in contact with other cities that were hit. He can't raise anyone in San Francisco at all. Carol asks if he's been able to contact Chicago but the answer is no. Attempting to be as positive as possible, Henry assures his neighbors that they are down but not out. If anyone needs food or water, the Abharts will share what they have. Henry will keep on trying to reach other cities.
At the gas station, Mike is only serving his regular customers. All others, especially those who never had time for him and Hiroshi, are ordered away at gunpoint. Carol wants to pay for her gas but Mike tells her it has already been paid for many times over.
Tom never turns up. Carol takes in Larry, a neighbor's child whose parents are missing. At a community meeting, the mayor, police chief, and Rev. Hollis are optimistic that help will arrive. There has been looting at the supermarket and pharmacy, but those places are now under guard. There are questions about exposure to radiation. Cathy Pitkin, a young mother and friend of the Wetherlys, is worried about the health of her baby. The doctor says that he doesn't have the equipment to measure how much radiation they've all been exposed to.
Food is rationed and batteries are collected to be given out as needed. Brad catches an older boy stealing batteries and confronts him. Gradually people begin to fall ill. Each day Henry Abhart is able to reach fewer cities. But life keeps going. The children give their play. Mary Liz continues her piano lessons with Henry's wife. Then the Pitkins' baby dies. They decide to leave and find a safer place, assuring Carol that they will come back one day. The doctor also leaves. There are rumors of survivors' camps in northern Canada, but the Wetherlys decide to remain in Hamlin.
Scottie falls ill and dies. Carol tears the house apart looking for his teddy bear but she can't find it. Rev. Hollis is unable to wait any longer and begins the service. Mary Liz asks her mother what sex is like, realizing that she herself will never know. Brad continues his daily visits to Henry, whose wife is now ill. He asks Brad to take over her job of checking on the neighbors.
Carol has been writing in her diary but can no longer remember the date. Larry dies and then Mary Liz. The boy who was stealing batteries breaks into the Wetherly home. Brad runs him off but the boy steals his bike. Reluctantly he begins using his father's bicycle, recalling the last morning they rode together.
A second community meeting is sparsely attended. Many have died. The police chief, visibly exhausted and ill, says that his men are on duty and will remain for as long as possible. Garbage pickup is suspended as they don't have the manpower. There is no longer any hope that help will arrive. Rev. Hollis urges people to bury the dead carefully as the cemetery is now full.
Hiroshi's father has died and Brad brings the boy home. Carol is now showing symptoms of radiation poisoning but manages to keep going. While she is watching a bonfire at the cemetery, Brad brings word of Henry Abhart's death. After he leaves, Carol begins screaming and pulling up chunks of grass. Rev. Hollis finds her and they kiss, not from love but the sheer need of human contact.
Carol and Brad decide to end their lives, taking Hiroshi with them. They close the garage door and climb into the car. Carol starts the engine, then cuts it off. She can't bring herself to go through with it. Brad also has changed his mind. It is his birthday. As a treat, Carol spreads graham crackers with jam and lights candles. Hiroshi suddenly brings her Scottie's teddy bear. She is overjoyed and asks where he found it but he can't tell her. Although they will all soon die, the mood is one of peace.