The end is the beginning. The beginning is the end.
Why it is hard sci-fi
A German series built around four families in a small town with a closed temporal loop spanning thirty-three years. The show commits to a single rule. The loop is consistent. Every generation s actions are the cause of the previous and the next generations actions. The shows graph of relationships is enormous and is mapped onscreen across three seasons. The shows physics is loose but the rule is rigorously enforced.
Science inside it
Closed timelike curves with a fixed thirty-three year period, a working Bootstrap paradox in which a notebook authored by no one is the source of the physics, a portrayal of branched-universe theory in the third season that treats two parallel worlds as a sustained interaction, and a final synthesis that closes both loops with a single intervention.
Spoiler alert
The third season reveals that the two interacting worlds are themselves a closed pair created by a single original world, and that the only way to close the cycle is to remove the moment of separation. The show ends with the two protagonists travelling to the original world and preventing the accident that created the two worlds in the first place. The show commits to the consequence. They cease to have existed.