There's a nuke in LA and Jack Bauer has to take it out. However, he needs his daughter in safety first, and another group of terrorists is planning to take out the CTU, where he works. In this episode, his daughter escaped with her employer's daughter, and arrived at the CTU just as it exploded. However, the CTU was evacuated as Jack sent a hostage (Jack was posing as a terrorist) to warn security. The building almost evacuates when the bomb goes off.
The big question is who survived. I know Tony survived, they had upcoming scenes from the next 24. But the nervous girl... she's in a coma, and if they wake her up, they get access to files, but she dies. If they rush her to the hospital, she lives, but puts all of LA at stake.
On top of this, the boss of the CTU, who was about to ditch town, just got radiation poisoning and is dead in a day. He was investigating leads and stumbled on where the bomb was constructed. Perhaps he's about to return, realizing he has nothing to lose.