The current president was working on a peace treaty with North Korea, but the National Assembly gets bombed along with most of the government. Our protagonist is the remaining designated survivor and becomes the acting president, and the first couple of episodes tackle him and the remaining survivors of the previous president’s staff trying to prevent the psychos in the military from starting a war - especially since during a state of war, control goes over to the Americans, which I wasn’t expecting the show to actually bring up.

The American commander even shows up demanding South Korea goes over to a more severe DEFCON level (and thus hand over control to the country), and rather un-diplomatically, openly declares “Your actions are in direct defiance of the will of Washington!” once they refuse.

The North Koreans initially refuse to communicate via the hotline between the two governments, which of course the military psychos use to support their intention for war, but it’s pointed out that it’s really the US and Japan (which sent a ship into Korean territorial waters) that are escalating tensions - why should North Korea be the one to de-escalate (and, given what was pointed out earlier, how can they even trust negotiations if it’s possible the Americans are actually in charge)?