What if you found a portal to a parallel universe? What if you could Slide into a thousand different worlds? Where it's the same year, and you're the same person, but everything else is different. And what if you can't find your way home?
Why it is hard sci-fi
A 1995 series in which a small group is stuck sliding between parallel Earths using a wormhole device with an imprecise return path. The show commits to one rule. They cannot get home reliably. Each episode is a different Earth with one or two altered axes, and the show treats the variation as a controlled experiment. The shows production budget falls off in later seasons but the early premise is rigorous.
Science inside it
Many-worlds branching with single-axis variations, a returnless transit problem, and a shared ledger of which Earth shows which counterfactual. The shows working idea is that a sufficiently large multiverse can be sampled by a small device and that no two samples will be the same.