Superior cold war paranoia which begins as strategic cat & mouse between a US destroyer patrolling international waters and a Soviet sub, then escalates into something far more deadly. Debut director James B. Harris usually worked with Stanley Kubrick, but this is more like documentary-realism than those films.
There are references to real life nuclear near misses and an authentic look, despite being mostly shot in a studio tank. Apparently this is Harris' response to Kubrick spoofing Mutual Assured Destruction in Dr. Strangelove (1964). Richard Widmark stars as the authoritarian skipper driven to the edge by misguided patriotism.
The suggestion is the relentless nuclear stand-off creates a unique pressure which will inevitably lead to mistakes. And accidents will happen; the final twist is pretty brutal. Sidney Poitier supports as the journalist who asks questions on behalf of the audience; this is only three years after the Cuban Missile Crisis.
And it's a bonus to see Eric Portman in an interesting minor role as a German veteran of the U-boats, now representing Nato. This is a UK/US co-production in b&w which feels like a glimpse into the secrets of cold war brinkmanship while also racking up considerable suspense from its conventional war-at-sea scenarios.