Lt. Cmdr. Randall Sullivan opens the story with a stark reminder of how deadly the Atlantic became when German U-boats formed relentless wolfpacks, sinking hundreds of Allied ships and tilting the balance of World War II. By 1943, Franklin D. Roosevelt and Winston Churchill make halting the U-boats the top priority, and with the United States fully committed to the war, Allied forces begin to claw back control of the sea.
In June 1943, Admiral Kentz discusses Sullivan’s upcoming mission, praising his leadership and suggesting Sullivan has something to learn. At home, Sullivan’s captain, Nathan Travers, promises his wife, Rachel Travers, that he will return safely, a vow that becomes a quiet thread through the perilous voyage. A couple of months later, Travers takes command of the fictional submarine USS Swordfish, based on the real vessel, with Sullivan at his side aboard a crew that constantly drills for every emergency point, ready to face whatever the sea throws at them.
Meanwhile, the German U-boat U-429, commanded by Jonas Herdt, survives a depth-charge onslaught and receives news that Hamburg has been bombed, a blow that hits him personally as he learns his daughter’s school was destroyed and there were no survivors. On the Swordfish, the mission’s tension grows as the XO, Teddy Goodman, becomes ill with a stomach rash that doctors fear could be meningitis, a highly contagious and sometimes deadly disease. Unbeknownst to the crew, Sullivan himself also bears a rash, signaling that he may be infected as well.
A second German U-boat, U-821, sinks a British merchant vessel, and Swordfish heads to investigate after there’s been little U-boat activity nearby. Radio operator Virgil Wright recognizes music from Glenn Miller emanating from U-821, and the Swordfish readies an attack. U-821 evades the first torpedoes, but the battle turns as Goodman dies from illness, and Travers presses on, taking command as the Swordfish fires a decisive third torpedo that cripples the U-821. The success is short-lived; the U-821 retaliates, delivering a torpedo that cripples and nearly sinks the Swordfish and forces Sullivan, Travers, and a handful of crew—Danny Miller, Ox, Benny Romano, and others including Wright, Abers, and Cooper—along with their German captors, to abandon ship and fall under the control of U-429.
The Germans split their prisoners between the bow and stern sections of the U-429. Wright tends to Sullivan, and Abers identifies Sullivan’s rash as meningitis, a revelation that casts a shadow over the prisoners’ prospects. The U-429, now crippled, is primed to attack the American destroyer USS Logan, prompting Travers and his group to improvise a ruse: they untether and fire a torpedo off course to signal their location, drawing Logan’s attention to the threat. Logan counters with depth charges, and both crews fight for survival, but Sullivan is killed while defending his men during a guard’s assault, a moment that marks a turning point in the mission and in Travers’s leadership.
The meningitis outbreak spreads, decimating two-thirds of the German crew, including Benny Romano. As Travers wrestles with the memory of his wife’s plea to return home, Jonas chooses to break the usual captor–prisoner rules and work with Travers’s men to reach the United States coast, hoping to spare more lives. He argues that he’s tired of war and wants to protect those who can go home, insisting that if they encounter enemies, their own men must still be allowed to return home.
During their voyage toward safety, Klause, the U-429’s quartermaster, grows disillusioned with Jonas’s defiance and allies with Bauer and Christophe to mount a mutiny. Abers and Wright subdue Christophe, and Hans neutralizes Bauer to protect Ox. Klause’s mutiny succeeds only in derailing the mutinous plan; Jonas, mortally wounded, bequeaths command to Cremer with his last breath, and the U-429 continues under Cremer’s leadership.
The U-429 again encounters Logan and the pursuing U-1221 moves in, firing torpedoes in a desperate bid to sink the doomed submarine. The stern tubes hold just one torpedo, and the decision is made to fire it where the enemy lies, but the detonation fails to deliver the decisive blow. The Logan, closing in with sonar, returns fire and destroys the U-429. In a final act of resolve, Travers, who has been wrestling with the choice to keep the Enigma out of enemy hands, disconnects from the Logan by faking a sinking to safeguard the submarine’s secrets and honor his promise to Cremer.
Back on dry land, Travers confronts Admiral Kentz, who acknowledges that the Germans remain enemies, but vows to do what he can to ensure they will be remembered and treated humanely. Travers and Rachel are reunited, and they visit Cremer in a POW camp. Rachel thanks Cremer for saving her husband’s life, and the two men exchange quiet, hopeful talks about a war that might soon end. Travers presents Cremer with cigarettes and acknowledges the sense of relief in seeing him once more, while Cremer watches with a hint of cautious optimism as the war inches toward its conclusion.
