J. K. Simmons is Howard Silk, an employee of the United Nations in Berlin, where he's worked there for 30 years. His job involves reading codes to another person through a glass shield, and recording the coded responses. But he doesn't know why. Until one day they drag him into a cold cement room and sit him down opposite a more badass version of himself (not really a spoiler, this happens in the first episode: if you know anything about the series, you know this). It turns out that there's a second Earth which appeared and started diverging from our Earth right near the end of the Cold War. The entry to that other world is a basement under the U.N. building in Berlin, and the U.N. has kept the whole thing secret while cautiously trading with the other world for knowledge, things like technologies that were invented in one world but not the other.
This is a Starz TV series, ten one hour episodes: there's also a second season that closes the whole thing out.
Tatiana Maslany rocketed to stardom on a similar premise: she acts as multiple clones of herself in "Orpan Black." I watched a couple episodes of that series and wasn't a fan - it wasn't about her, I didn't like the setup. She was good, but she did it by exaggerating character tics. J. K. Simmons on the other hand ... here's the genius of his performance: I don't know how he's doing it. 90% of the time when Howard walks on screen, you know which one he is by a glance at his face. We're not talking about clothes here (although they play a part) or the setting (although it sometimes helps), it's just ... his face. The two Howards have an identical history for about half their lives, but they diverged significantly after the worlds split. And those differences of opinion and experience show.
The setup is entirely and intentionally reminiscent of the Cold War. The two worlds are trading, but there's bad blood and political manoeuvring and spying. All of which happens in Berlin - you know, where the Wall was? Mostly in English, some subtitled German. The simplicity of the effects (two J. K. Simmons and a couple other people, not much else) is in direct contrast to the complexity of the script: there's a lot happening, it's well thought out, and you need to pay attention. I enjoyed the first six episodes more than the latter four: in the seventh episode we go to "The School," which takes us away from Simmons' great acting for an episode and also starts us down the path into a more violent closure of the season. To that point there had been some deaths and it was tense, but that was the turning point where it became more overt.
Overall an outstanding piece of work that should be sought out by any fan of science fiction. Or acting.
SPOILER ALERT, don't read the rest of this if you haven't seen the season, etc. The one complaint I have is that Berlin on Earth Prime has an abundance of new, fancy architecture. This is probably just to differentiate the two Earths, but the rest of it is so well thought out ... and the reality would be that they wouldn't have the cash to build new buildings, nor the people to occupy them, when 7% of your planet's population is dead. They'd be in a massive economic slump ...