Thunder Afloat

In a New England port town, Pop Thorson and Rocky Blake are rival tugboat owners. Thorson's boat sinks while docked, and he suspects Blake sabotaged it to win a lucrative government contract. Susan Thorson, his daughter, tries to keep peace between them while balancing loyalty to her father and her affection for Blake.

As the U.S. moves toward entering World War I, Susan and Pop Thorson trick Blake into enlisting by pretending it was Thorson who would enlist first. Blake joins the Navy, bringing a sense of duty to the looming conflict. This sets the stage for their evolving dynamic under war pressure.

Thorson wins the contract to move military barges, but his boat is sunk by the German submarine while towing a barge. The German crew orders everyone into lifeboats and sinks the tug and cargo, creating a personal stake for Thorson in the war. He vows to take his place in service anyway.

Thorson reaches port and enlists in the Navy, quickly being made an ensign and placed in command of a small sub chaser. Blake, now a lieutenant, is his superior and dedicated to enforcing naval discipline. The old rivalry resurfaces under the pressure of wartime duty.

On their first patrol, Thorson reasons a submarine would hide in the lee near Nantucket Island and heads there alone on the orders he ignores. He does locate the same German submarine, but the enemy tricks him by releasing oil and sinks a lightship, then escapes. A damaged cable fouls his propeller, forcing him into the water where Blake must rescue him.

Back at port, Thorson is court-martialed for insubordination and demoted to ordinary seaman, assigned to shore duties. Despairing, he contemplates deserting to Canada with Susan, but Blake intervenes and gives him a new assignment that could still honor his wartime skills.

Thorson is tasked with serving aboard a decoy fishing schooner, whose civilian disguise hides a concealed radio to summon sub chasers. The decoy fleet joins a real fishing fleet to attract enemy submarines. It is a high-risk tactic born of desperation and wartime improvisation.

The decoy is attacked by the same submarine, and the Germans board to search for the radio. Before they can capitalize, Blake's awaiting fleet moves into position. The board reveals a critical vulnerability, raising the stakes for the next engagement.

With the captain's blessing, Thorson orders the decoy to ram the submarine. The German crew sinks the decoy and takes Thorson aboard as a prisoner before diving to the seabed. He remains a hostage while the surface patrols close in.

Biding his time, Thorson bangs on the hull with a wrench to alert the listening sub chasers to the submarine's location. His improvised signal gives Blake's fleet a chance to strike. The tension hinges on this risky act of ingenuity.

Blake chooses to attack the submarine despite the danger to Thorson. The sub is damaged but manages to surface, and Thorson and his crew are taken off. The rescue completes a risky turn in the war's small-scale combat.

Thorson is awarded the Medal of Honor and restored to the rank of ensign. He resumes command of a sub chaser, much to Susan's relief and the satisfaction of the port community that has followed his trials of duty and loyalty.

With his past struggles behind him, Thorson returns to active duty in command of a sub chaser, supported by Blake and Susan. The film closes on a note of renewed trust among friends and a town celebrating a wartime hero.