I will say right up front that Dark is not an easy show to watch -- but that shouldn't deter you! The show itself does everything it can to make it clear who everyone is, using some friendly split-screens a few times to show patterns and equivalencies between past, present, and future. But there are so many characters, some of them even the same person at different ages, and more than once you'll be impressed at the casting department's ability to find actors who genuinely look like they could be younger and older versions of each other. The story gets more intricate by the episode, introducing so many new time periods and interwoven character connections that, even if you watched the first season, you'll probably want to refresh your memory, either by watching it again or reading a few episode summaries. If you're feeling overwhelmed, there's always the very handy Dark official website, which has information in every character, time period, special object, and notable place (like, for example, the nuclear power plant looming over Winden, which makes watching this in the wake of HBO's Chernobyl kind of funny).
The show is also very slow in comparison to zippier fare like Stranger Things, but you'll be thankful for its contemplative pace when you're four episodes deep and getting tangled in the wads of yarn on the conspiracy wall you're constructing in your bedroom. In Season 1, you watch how everything begins, from one boy's disappearance to what looks like a nuclear apocalypse. In Season 2, everyone starts to figure it all out. There are multiple time travelers visiting their parents, their grandchildren, their own pasts; competing factions warring for dominance over time itself, kidnapping and murdering for their cause. A certain book with clues about how to do time travel only exists because someone from the future brought it back to the past. Another character realizes that even if he manages to set everything right, it would mean that he would never be born. You get people meeting their younger or older selves, dealing with the frustration of being unable to explain what's going to happen for fear of changing the course of history.
The show is also a crash course in how time travel would, hypothetically, work, gradually and deliberately outlining various paradoxes and theories and using the residents of one small town to act them out. Once time travel is invented, time travel has always been invented. Everything that happens is now a set series of events that can't be changed -- must not be changed -- lest some unknown catastrophe occur, which obviously brings the issue of free will front and center.
It's also a really affecting study of the fatalist trope of children being doomed to re-enact the sins of their parents. This is a German show, set in Germany, where the concept of sin and doomed repetition still has all-too-relevant reflections in the real world. World War II isn't explicitly mentioned -- given the 33-year cycles, it's actually skipped over completely -- but it's there if you look for it.
How are we supposed to grow and change and become better when there is literally nothing we can do to stop the bad things from happening? The crises that rock Winden come in waves -- every 33 years, coincidentally -- and Dark gives those crises a haunting, Biblical flavor that sticks in your gut long after the novelty of its thrilling sci-fi aspects wears off. If you're looking for meatier, more interesting fare than, say, a certain '80s teen pastiche, Dark is worth a binge.