It's 1961 and Russia is anxious to make a statement of deterrence to the U.S. with the introduction of their latest nuclear submarine, the K-19. Unfortunately, poor construction and supply efforts have left Captain Mikhail Polenin (LIAM NEESON) with an unfinished sub and inability to successfully complete pre-launch drills.
This forces Kremlin official Marshal Zelentstov (JOSS ACKLAND) to send revered Captain Alexei Vostrikov (HARRISON FORD) to command the K-19. This doesn't sit well with the crew or its officers including Yuri Demichev (STEVE NICOLSON) who tell Polenin that he's still their captain despite being bumped to Executive Officer.
Vostrikov immediately ruffles feathers with his stern attitude and demanding behavior that includes replacing the senior reactor officer with Vadim Radtchenko (PETER SARSGAARD), a young graduate of the nuclear training academy who's never served on a sub, but will now be working with Pavel Loktev (CHRISTIAN CAMARGO) in watching over K-19's reactors.
Despite the fact that various crewmembers now refer to the sub as the "widowmaker" - when considering that various men have already been killed before the initial launch - Vostrikov sets sail with the goal of test firing a nuclear missile. After some tense and suspenseful moments involving repeated drills, the crew is successful and then ordered to head to the American coast.
Yet, part of one of the sub's nuclear reactors then breaks, thus raising the core temperate to dangerous levels that could result in a thermonuclear explosion. With replacement Dr. Savran (DONALD SUMPTER) not being knowledgeable about how to deal with the resultant radiation burns afflicting those trying to repair the problem, and radiation levels progressively increasing inside the sub, Vostrikov and Polenin end up at odds over the proper course of action to deal with the worsening situation.