24 is a thriller which is shown in "real-time," with each minute of airtime corresponding to a minute in the lives of the characters. Watched continuously each season would take approximately 18 hours to finish. This real-time nature is emphasized by an on-screen digital clock appearing before and after commercial breaks, with a distinct beeping noise for each second, alternating between C and D tones. This time corresponds to the in-universe time of the show. The characters will often place time windows (such as the common "within the hour") on certain events such as terrorist threats, thus strongly hinting the attack/event will occur before the end of the episode.
The action switches between different locations tracing parallel adventures of different characters involved in the same overarching plot. As a result, there may be long sections of unseen narrative for each character, in which case a character may only be seen for a portion of an episode's overall running time.
Season 1
Season 1 begins and ends at 12:00 AM, and occurs on the day of the California presidential primary. Jack Bauer must protect Senator David Palmer from an assassination plot, and rescue his family from those responsible for the plot, who seek retribution for Jack's involvement with a covert U.S. mission in the Balkans.
[edit] Season 2
Set 18 months after season 1, season 2 begins and ends at 8:00 AM. Jack must stop a nuclear bomb from detonating in Los Angeles, then assist President David Palmer in proving who is responsible for the threat.
[edit] Season 3
Set 3 years after season 2, season 3 begins and ends at 1:00 PM. While struggling with a heroin addiction, Jack must re-infiltrate a Mexican drug cartel in order to acquire a deadly virus being marketed underground. Jack must then stop the mastermind behind the virus from releasing it.
[edit] Season 4
Set 18 months after season 3, season 4 begins and ends at 7:00 AM. Jack must save the lives of Secretary Heller and his daughter Audrey Raines (with whom Jack is romantically involved) when they are kidnapped by terrorists. The same terrorists then launch further attacks against America, and Jack is forced to use unorthodox methods to stop them, methods which will have long-term consequences for both Jack and the U.S.
[edit] Season 5
Set 18 months after season 4, season 5 begins and ends at 7:00 AM. Jack is believed to be dead by everyone except a few of his closest friends. Terrorists with connections to the U.S. government attempt to steal nerve gas in order to protect US oil interests in Asia, and Jack must resurface to stop them as well as dismantle the government conspiracy.
[edit] Season 6
Set 20 months after season 5, season 6 begins and ends at 6:00 AM. Jack is released after being retained in a Chinese prison for twenty months. Terrorists plot to set off suitcase nuclear devices in the United States and Jack must stop them; after that, Jack has to prevent sensitive circuitry from falling into the hands of the Chinese to prevent war between the U.S. and Russia.