SEASON 1Jack races to thwart an assassination attempt on presidential candidate David Palmer on the day of the California primary, while also trying to rescue his kidnapped wife and daughter. Co-creator Robert Cochran ''I was proud of the first 12 episodes, but I feel we sort of repeated the story in the second half of the season: We kidnapped people, we tried to kill Palmer again. It would've been nice to come up with different stuff.'' Co-creator Joel Surnow  ''I do think we acquitted ourselves at the end by killing [Jack's wife] Teri. That really defined the show. This is a tragedy, this is not a Jerry Bruckheimer-rah-rah-the-good-guy-wins show. [Jack's ex-lover/fellow agent] Nina being the mole and Teri getting killed were a one-two punch second to none.''

SEASON 2Jack must stop terrorists from detonating a nuclear bomb. Surnow ''We realized the show is even better if Jack is saving a lot of innocent people, not just a presidential candidate.'' Cochran ''That's my favorite season. I thought it was cool that we set off the bomb, because that's another thing you don't think is going to happen.''

SEASON 3Jack tries to prevent a Mexican drug kingpin from selling a weaponized virus. Surnow ''Season 3 was problematic because we kept a secret from the audience for the first seven episodes [that Jack and two CTU agents were undercover with a Mexican cartel]. I don't know if it paid off as vividly as we wanted.... However, [after] the virus got unleashed in the hotel, that was some of the best stuff we'd ever done. That was a chilling depiction of what happens hour by hour in a place where a weapon got out. This is the first time the terrorists actually scored.''

SEASON 4Jack combats a sleeper cell that plans to attack nuclear power plants. Cochran ''If you laid out the stuff we did, it sounds ridiculous. We kidnapped the secretary of defense, melted down nuclear power plants, stole a stealth bomber, shot down Air Force One, and used a nuclear football to get a warhead and fire it, all in 24 hours. I probably left out some stuff. But it worked fine.''


Sign up for EW.com's What to Watch Newsletter!

What to watch on TV. Hear what's on tap for the night ahead and get witty, morning after recaps of top shows (sent weekday mornings).