The story is obviously based on a historic event: the voyage of the submarine U-234, which was bound for Japan. It had weapons, detail drawings for weapons, and uranium aboard. The uranium was meant to help Japan build an atomic bomb. The sub started in April 1945. When Germany capitulated in May, they had to decide whether they would go on to Japan (who hadn't capitulated yet) or surrender...
The movie isn't absolutely accurate historically, but that's not a big problem in my opinion. The makers don't pretend to be accurate; the sub in the movie isn't called U-234. The plot is the big problem. There are a number of small inconsistencies, for example when there's a leak, one of the two Japanese officers aboard seems to know how to repair the (German) sub better than the Germans themselves. I think I could live with these small inconsistencies, but there's also one plot hole that is deeper than the Mariana Trench (I've also sent this as an update to IMDb's 'goofs' section for this title; hope they'll accept it): At the beginning of the movie, one of the German officers explains the abilities of the sub: it can stay submerged for a very long time. As long as it's submerged, detection of the sub is difficult. So why do they stay on the surface almost all the time? This makes the captain seem ridiculously stupid. Why doesn't he take advantage of the best feature of the sub? Of course, they are detected by the radar of a British destroyer (but that is not the end of the movie; I won't give away the ending because I don't want to spoil it.)
On the other hand, I liked the part when they have to decide whether they want to go on or surrender. The officers have differing opinions, there are heated discussions, tension builds. And I think the actors did a good job.
But, as I've said before, it could have been a lot better in my opinion, so I have given it only six points. There was a documentary about U-234 on German TV. Its title is 'Kurs Fernost', made by Discovery Channel. I don't know the English title or if it has ever been shown outside of Germany. It's very good and interesting in my opinion. If you are interested in the theme and have the chance to see it, I think you should consider watching it. And if you want to see a really good submarine movie I think I can recommend the mini-series version of 'Das Boot' (it's longer than even the director's cut version).
