I think this is on AMC in the US although I've been watching on Netflix as they have bought the international rights (as an aside it will be very cool if this becomes common).
The basic premise is that the President, his cabinet and all of Congress are killed in a huge terrorist attack on the Capitol at the state of the union address. Kiefer Sutherland plays the Sec for Housing and Urban Development and as the Designated Survivor (not sure if this is a real thing) during the attack is sworn in as President. The story is basically the simultaneous attempt to rebuild the government as well as investigating who committed the attack and why.
I've really liked it so far. Anyone else watching?