In this season, Jack Bauer (Kiefer Sutherland) quits the drug addiction he contracted in the third season, and leaves CTU to start working for the Secretary of Defense. But Jack can't get rid of his connection with CTU. At the beginning of the series, a train derailment caused by a terrorist attack forced CTU to recall Jack to investigate the matter. Not long after investigating the matter in depth, Jack discovered that the next target of the terrorists was the Minister of Defense. After Jack's efforts, the kidnapped Minister of Defense was rescued and the crisis was resolved. Jack, who originally thought he could breathe a sigh of relief, discovered that the assassination of the Minister of Defense was just a small link set by the terrorists to disrupt the CTU investigation. The real purpose of the terrorists is to control nuclear power plants across the United States, and to threaten the entire United States by causing nuclear leaks by overloading the nuclear power plants. The crisis is really unfolding...