TV Guide says: "Following a lead on a company that made nerve gas for the military, Jack encounters an old nemesis. Meanwhile, terrorists plan an attack."
The nemesis is clearly Christopher Henderson, the guy who Jack asked Audrey about last episode. There are plenty of spoilers online about who exactly Henderson is, and who plays him, but I'll let you look for those on your own if you're curious.
So - Will Martha Logan get blown up? Will Jack scowl? Will Chloe smile? Where did they find an actress that skinny to play McGill's crackhead sister? Is she a real crackhead? Some of these questions answered tonight.