A group of men are searching for something of which there are many, but are not easy to find.
The crew are hunting for allied and merchant shipping, which there was a lot of, but which is actually very difficult to find and intercept.
A number of them do not speak english.
In fact, they speak german...!
They become trapped in a steel container which occasionally makes noises which worry them, and become very tense, as well as diseased.
The steel container is the submarine, the desease is crabs, which they all catch from close proximity for protracted periods.
At times they are frightened and one of them is almost driven mad, almost needing to be shot by another man.
The chief engineer goes nuts after a particularly deep dive
They occasionally receive messages from the outside. After running out of food and water, they manage to leave the container, but must return to continue their search after gathering more provisions.
At one point in the film they are resupplied
The pressure begins to get to them, and they resign themselves to death.
This was a subtle clue about it being a submarine film - pressure...underwater. They become stuck in the straights of gibraltar and the U boat begins to break apart
Miraculously, they escape the box a second time, but many of them are killed before the end of the film.
An allied air raid after they have docked sinks the boat and kills most of the crew at the end of the film.
