Dark (2017)

It opens as a classic small-town disappearance: a boy called Mikkel vanishes near the caves outside the German town of Winden, and four families who have known each other for decades start watching one another. Then the show reveals what kind of mystery it really is, and the search for one missing child turns into a puzzle spread across generations. Pick it if you like keeping notes and family trees, because Dark asks for full attention. The payoff is that every family secret ends up mattering. It was Netflix's first German-language original, ran three seasons and finished on its own terms, so you can commit knowing it has a real ending.