Fear of the unknown propels Jericho into social, psychological and physical mayhem when all communication and power is shut down. The town starts to come apart at the seams as terror, anger and confusion bring out the very worst in some residents. Jake Green, the prodigal son of the town's mayor, becomes a reluctant hero when a school bus crashes as a result of the explosion.

With only an hour before the fallout from the nuclear blast in Denver hits Jericho, Jake rushes to get the townspeople inside and away from the rain so that they won't get radiation poisoning.

Jake organizes a group to gather more information about the explosions after a satellite television in the local bar broadcasts an Asian news program showing the extent of the nuclear attacks. In addition to the television broadcast, Stanley, who is hospitalized for radiation poisoning, claims to have seen tanks headed toward Denver. With Jericho still cut off from communication and the people craving information, Jake recommends sending out four search parties in different directions to get a sense of what is going on outside of town. Meanwhile, when one of the inhabitants dies unexpectedly, Heather questions the cause of his death.

When Jake and Heather find a stranger on the side of the road who is sick and disfigured from radiation poisoning, they must decide whether to help him and risk contamination or do nothing and allow him to die. After they decide to help the stranger and take him to the hospital, Jake and Hawkins sit vigil in hopes that he will be able to give them information about what's going on outside of Jericho. Meanwhile, when the generator at the hospital is low on gas and supplies in town begin to dwindle, the police must ask disgruntled and panicked townspeople to siphon gas from their cars.

The people of Jericho receive a pre-recorded phone call from Homeland Security telling them that help is on the way. The townspeople start to think that life might be returning to normal when the power and phones return. Their hope is dashed, however, after the power surges and electrical lines go down, causing fires to break out all over town. While, when Jake and Hawkins work together to fight the fires, they discover that neither one of them is who they claim to be. Meanwhile, still desperate for information, the townspeople vigilantly watch the satellite television in Mary's bar with the hope that another signal will come through.

An electromagnetic pulse fries everything with a circuit board at 9:02 pm, plunging the town into darkness. Before the power goes out, the citizens of Jericho watch as minutemen missiles are launched into the sky. One by one, the townspeople receive phone messages alerting them that the Federal government has been shut down and that everyone is under local or state jurisdiction. For the first time since the explosion, people realize they are on their own and must be responsible for maintaining law and order in their town. Meanwhile, Allison, Hawkins’ daughter, starts to question her father's motive for moving to Jericho.