It is VERY good.
The premise (Explained early in episode 01 and expanded upon through the series) is that in the 1980's german scientists caused an quantum accident that created an alternate world, identical in every way (we don't know if that world is outr or the other one). The result is that the two worlds are connected through an tunnel in Berlin, and a secret international organization keeps the tunnel an secret and manages relations between both sides.
Differences in the two TL's are actually quite interesting points. Worldbuilding spoilers ahead. 9/11 didn't happen in the Alternate world, however in the late 1990's a massive flu pandemic killed about 5% of the world population. This is a major change that resulted in the Alternate timeline being significantly behind in many technologies( smartphones are not a thing), while being ahead in others (medicine is WAY more advanced). The alternate timeline is also very paranoid, because, considering nothing nearly as major as the flu hit our world, and the POD was still so young, some suspect the flu might have been an biological attack from our world.
The series is excellent, and deals with themes like Espionage, identity, diplomacy, etc, in clever ways. The incredible cast helps.