The one hundred first TV show: #362 Das Boot
Das Boot is a war film... up to a point. While set on a submarine in the second world war, aside from some short sequences at the start and end, and one refuelling stop, the crew is isolated. They may hear bits of news over the radio, but they only have themselves to rely on. Even a passing sub in the distance is something you can only wave at and send light signals to, you can never really get close.
It creates camaraderie and tension and while the movie plays with that, too, the bigger threat is the danger of the environment. Dive too deep and the sub can't take it - you need to patch it up and fix things. You're mostly defenseless against a bunch of threats. You're a lonely island to defend the mainland, but can't go anywhere. It creates a psychological drama that's intense already.
Then there are the attacks. The ships and planes that do are faceless - frightening big things you'd like to take down but that harm you just as much. The sub recovering from those and the crew pulling together to do so lead to some of the most harrowing, challenging scenes while also feeling genuinely tense.
The tension comes from the filming environment as well. Everything is small and constrained and even the camera has to carefully weave around everything, as the set was built to scale. It's expertly done, but you feel that oppression at all times. The atmosphere comes across in every scene and these six episodes felt like they flew by because of it.