Imagine if the unimaginable happened: every member of the federal government is gathered in a single location, and there's a nuclear strike. Or a massive earthquake. Or a meteor impact. It would kill everyone in charge of leading the country, and the government would unravel. Luckily, the U.S. government has thought of a solution for such a problem: the designated survivor.
This may sounds like something out of a TV show, and, yes, it's the basis of the drama Designated Survivor, but that show is based on a very real concept. Before an event where the country's leadership will converge in one location—such as The State Of The Union or a presidential inauguration—the President and top presidential aides choose someone to skip the event, just in case catastrophe strikes.