In this episode, we get answers on just how bad the problem of far-right infiltration in the German military and police really is — and how Germany is trying to address it. We learn about Germany's "defensive democracy," which was designed after World War II to protect the country against threats from the inside. One of those threats, according to some German officials, is the Alternative for Germany, widely known by its German initials AfD. We meet intelligence officials who have put parts...

We meet Franco A., an officer in the German military who lived a double life as a Syrian refugee and stands accused of plotting an act of terrorism to bring down the German government.

Franco A. is not the only far-right extremist in Germany discovered by chance. For over a decade, 10 murders in the country, including nine victims who were immigrants, went unsolved. The neo-Nazi group responsible was discovered only when a bank robbery went wrong. In this episode, we ask: Why has a country that spent decades atoning for its Nazi past so often failed to confront far-right extremism?

The mysterious story of a German soldier, a faked Syrian identity and a loaded gun in an airport bathroom cracks the door open to a network of far-right extremists inside the German military and the police. They are preparing for the day democracy collapses — a day they call Day X. But just how dangerous are they? See all episodes of Day X at nytimes.com/dayx

What starts with a mysterious gun in an airport bathroom and a fake refugee identity opens the door to a nationwide network of far-right extremists inside Germany’s military and police. It’s a story about a changing national identity — and the backlash against it — raising a question that democracies across the world are waking up to: What happens when the threat is coming from within?