A sequel to Down Periscope, where Tom Dodge is soon to retire, and his Seawolf sub too (which is accurate enough, counting years that have passed since the original, if they started soon they could have it out for 2015, 20 years since the original).
In the plot, he's done a lot of things, but nothing that really got him a high honor/medal/award.
The Navy are testing a prototype ('borrowing' from the film 'Stealth') AI submarine system, and although they take the precaution of putting the systems in a slower random diesel sub, the fact that all the now unneccessary bunks/cabins/'furniture' has been removed/cut out make the sub lighter and faster than standard.
Inevitably, out at sea, the diesel malfunctions and disables the nuclear sub accompanying it and possibly a warship as well, and the next closest sub that can do anything about it is Dodge's.
So he and the crew (Sonar and a couple of other characters return) have to chase down the diesel. The use of a diesel as the AI sub is crucial because they look meaner and uglier than the nuclear ones, almost as though they have a face where the tower is.
And I'm not sure if the accuracy is there (I think the harbor might be too shallow?), but I was thinking that as Dodge starts closing in on the sub, it manages to sneak into the Shanghai harbor/shipping lane, and goes under the ships in order to get away.
Dodge is stuck outside the harbor awaiting official clearance to enter (not telling them the real reason they need to enter). Meanwhile the Chinese navy discovers the rogue diesel on their radar/sonar systems and pursue it themselves, so now Dodge has to get to it before they do and try to stop an international incident of sorts.
Dodge of course manages to get to it but only after a Chinese sub gets roughed up by it (one of their crew gets injured, and the chinese crew gets plenty of screentime/have characters - important for later), and had one of his crew makeshift a normal torpedo into being a magnetic one (attaching magnets to it/stroking it with powerful magnets? I don't know torpedos lol) as they chase the diesel through a curvy river system. (there needs to be suspense for if the torpedo was going to hit or not).
The torpedo hits, and we get the usual mission complete/hooray stuff, and during that, they've gotten up on deck, and after that, they turn around and see a mass of Chinese navy ships lined with personnel on deck, sitting idly.
There's an awkward face to face moment between Dodge and the main Captain of the Chinese navy, and then the Chinese Captain smiles/laughs/says something and all is kinda forgiven (there's been top level talks).
And then there's a 'but what of the submarine' thing between both Captains, and then the Chinese captain says he thinks he might have an idea, and lets his injured crew member call the shots to torpedo and finish off the diesel.
I don't want to go cliche here, but I think Chow Yun-fat would go well as the Chinese navy Captain, and help 'bring' his crew's characters into the film/let the audience know that they are significant/not minor characters that won't be seen again.
At the end, Tom Dodge gets awarded the medal of honor or something.