On Christmas Eve, the submarine Copperfin, under the command of Captain Cassidy, slips out from Mare Island Naval Shipyard on a secret mission with a tightly focused objective. Their orders direct them first to the Aleutian Islands to rendezvous with a PBY Catalina and bring aboard Raymond, a meteorologist, before steering toward Tokyo Bay to secure vital weather intelligence for the upcoming Doolittle Raid.
After picking up [Raymond], the Copperfin faces sudden danger as two Japanese floatplanes close in. They shoot them down, but one pilot somehow survives and parachutes into the water. When Mike Conners volunteers to pull the downed aviator aboard, he is fatally stabbed by the wounded pilot. In the aftermath, Tommy Adams, the young recruit, fires at the intruder, yet his hesitation weighs on him. To seek redemption for his delayed reaction, Tommy Adams volunteers to defuse an unexploded bomb lodged beneath the deck, a dangerous task under pressure and with limited ether and makeshift tools at hand. The crew holds a burial at sea for Mike, and the mood aboard darkens as the ship faces the cost of war.
In the wake of the tragedy, Tin Can keeps a stubborn vigil about not attending the service, until he explains that every Allied death inflicts him deeply. Nearby, Raymond reveals a personal perspective: having lived in Japan, he offers a pointed view on how the war was driven by the country’s military factions and the beliefs that steered its people toward conflict.
As the Copperfin nears Tokyo Bay, the crew must thread through defensive minefields and anti-torpedo nets. When a Japanese ship enters the harbor, Captain Cassidy seizes the moment and follows in its wake, threading a dangerous path toward observation. That night, a three-man shore party, highlighted by the roguish wit of Wolf, goes ashore to make weather observations and gather crucial intelligence without tipping off enemy watchers.
Back aboard, Tommy Adams endures a painful bout of appendicitis, and Pills, the pharmacist’s mate, must perform an improvised operation using limited tools and without enough ether to endure the procedure. The patient survives, and Cookie Wainwright sets about fulfilling a promise to bake a pumpkin pie for Tommy, adding a rare touch of warmth to the tense mission.
Raymond, in his efforts to stay hidden, broadcasts the weather observations in Japanese to mislead any potential eavesdroppers. The ruse buys them time, but the Japanese are not easily outsmarted, and search efforts intensify around the bay. Yet the Copperfin remains undetected, allowing the crew to glimpse a portion of the Doolittle Raid through the periscope and to feel the weight of the looming air assault on distant shores.
After collecting the critical meteorological data and safely bringing Raymond and his team back aboard, the Copperfin slips out of the harbor, threading a cautious escape as the anti-submarine nets open to let it pass. The battle-scarred submarine then closes in on its return journey, its crew battered but stubbornly resolute.
In a final, furious confrontation, the Copperfin strikes a Japanese aircraft carrier, taking heavy damage from its escorts. With the clock ticking and depth charges pounding the hull, Captain Cassidy hammers home a desperate gambit, sinking a destroyer in a bold move that buys precious moments for the crew to fight their way home. Exhausted but unbroken, the Copperfin emerges from the long, dangerous night, carrying the experiences and memories of a crew that faced treacherous seas, hard choices, and the relentless demands of war.
