The first season of the American drama television series 24, also known as Day 1, was first broadcast from November 6, 2001, to May 21, 2002, on Fox. The season's storyline starts at midnight and ends at the following midnight on the day of the California presidential primary.

The season's main plot revolves around an assassination attempt on David Palmer, a U.S. Senator from Maryland who is a presidential candidate for the Democratic Party, on the day of the primary election in California. The central character is Jack Bauer, a former United States Army Delta Force operator, who is the Director of the fictional Counter Terrorist Unit (CTU) in Los Angeles. Bauer becomes professionally as well as personally involved when his wife Teri and daughter Kim are kidnapped by the people behind the assassination.

The season is divided into two halves, each with a distinct villain. The first focuses on a mercenary group, led by Ira Gaines, in their efforts to control Bauer by kidnapping his wife and daughter, blackmailing him to kill Senator Palmer. The second half focuses on the Drazen brothers, who hired Gaines and his men, as they execute a contingency plan. As CTU tracks their activities, Bauer learns why he and the Senator are being targeted.

Fox initially ordered only 13 episodes; the successful rescue at the end of the season's first half was planned as a series finale in the event that the show was not renewed.

The show follows each of the five main characters—Jack Bauer, Teri Bauer, Kim Bauer, Nina Myers, and David Palmer—simultaneously, as well as one main villain: Ira Gaines, Andre Drazen, and finally Victor Drazen. The day's situations and the relationships between these five characters and their associates form the basis for the season's subplots, which include:

Season 1 starts and ends at 12:00 a.m. PST on the day of the California Presidential Primary. The season focuses on presidential candidate David Palmer, and the repeated attempts on his life prevented by Counter Terrorist Unit agent Jack Bauer, who becomes personally involved when the assassins kidnap his wife Teri and daughter Kim in an attempt to blackmail Bauer.

The first 13 episodes focus on the efforts of a terrorist cell led by Ira Gaines to assassinate Palmer. Bauer becomes an instrument for Gaines after Gaines kidnaps his wife Teri and daughter Kim in order to force him to aid in the assassination and take the fall for Palmer's death. Forced to go rogue from the agency, Bauer gets assistance from Nina Myers and eventually Tony Almeida as they attempt to identify a mole in CTU helping Gaines.

Throughout the day, Palmer also deals with his ethical responsibilities as it is revealed that his son is implicated in the death of his daughter's rapist. He clashes repeatedly with his wife, his son, and his fixer trying to figure out the best way to handle the coming story. Upon discovering his wife Sherry's role in the cover-up, he begins to doubt her motivations. After the first assassination attempt fails, he remembers Bauer, and eventually the two meet and begin to work together to take down the men who hired Gaines, Andre and Alexis Drazen.