The skipper of a sunken ship stands trial for negligence.

On board the salvage boat Sea Witch in the storm-tossed English Channel, John Sands and his partner Mike Duncan receive notice of a gale warning, then moments later, find themselves in the path of an oncoming ship. When their frantic signaling goes unanswered, Mike and John manage to steer the Sea Witch out of the vessel's path. John identifies the ship as the Mary Deare and is perplexed that the bridge appears unoccupied. Hoping to claim the ship as salvage if it is abandoned, John boards the Mary Deare and finds the bridge empty, but is intrigued by a map with the current course plotted. In the captain's quarters, John locates the cargo manifesto which lists the ship's freight as tea, cotton and airplane engines and finds several letters identifying the ship's commander as Capt. Taggart. Going below, John finds the bulkhead damaged and leaking profusely, then is startled by the appearance of an exhausted, bedraggled man. Upset when John addresses him as Capt. Taggart, the man identifies himself as Gideon Patch, the former first officer who took command when Taggart died unexpectedly four days earlier. When John advises Patch that the Mary Deare is drifting toward the dangerous French Les Minquiers coral reefs, known to the British as "the Minkies," Patch orders him off the ship. Although puzzled by Patch's behavior, John returns to the deck where the gale is blowing near full strength. As Mike struggles to hold the much smaller Sea Witch alongside the Mary Deare , John slides down a rope, attempting to swing across to his boat, only to be continually buffeted violently against the side of the bigger ship. When John begins slipping down the rope, Patch pulls him back aboard the Mary Deare and Mike sets off for the nearest port. Under John's prodding, Patch reveals that he has been unable to signal for help because of the destruction of the radio room in a fire earlier. Patch then declares he believes the same person who set the radio room on fire tried to knock him out, but refuses to elaborate further. Frustrated by Patch's strange account and inaction, John goes to the boiler room to attempt to build up enough steam to steer the drifting Mary Deare away from the Minkies. After stoking the boiler room fire all night and helping start the engines, John is shocked to discover that Patch has plotted and steered a course directly into the Minkies. When John accuses Patch of deliberately attempting to wreck the Mary Deare , Patch orders him to continue stoking. Confused and unable to handle the ship alone, John complies and Patch drives the Mary Deare aground on the Minkies reef. The next morning, as the men prepare to sail the lifeboat to St. Malo, Patch tells John that the Mary Deare was badly damaged by two mysterious fires and an explosion and that he was knocked unconscious just before the crew abandoned ship. Patch hopes that an independent company will be appointed by a court of inquiry to examine the ship. Later, after a rescue plane spots the lifeboat, Patch pleads with John not to reveal the location of the Mary Deare until the inquiry. Reunited with Mike at St. Malo, John learns that years earlier Patch was involved in the loss of a ship, resulting in a suspension of his master's ticket for several months. That afternoon John, Patch and the hostile Mary Deare crew are questioned by the insurance company's underwriter. When first mate Higgins insists that Patch panicked during the second fire and gave the abandon ship order, Patch angrily knocks him down. Upon returning to South Hampton, John, Mike and Patch are met by Mr. Petrie, a representative of the ship's owner, Mr. Gunderson. Petrie questions John about the location of the Mary Deare , but John remains evasive. The following day, Patch travels to London to meet with Taggart's daughter Janet who admits that although she hadn't seen her father in several years, they corresponded regularly. Patch asks if Janet's father wrote her from Rangoon and explains that he boarded the Mary Deare after the ship left Rangoon, as a replacement for a sick officer. Janet reads Patch the letter which reveals that the Mary Deare was docked in Rangoon for four days next to another ship from China, whose captain Taggert knew well. That night, John prevents Patch from sailing on a rental boat and promises to remain silent about the Mary Deare only if Patch vows not to return to the wreck on his own. The next week at the court of inquiry, during questioning, Patch testifes he believes that the fires and explosion were part of a plan by the crew purposely to sink the ship to collect insurance on the cargo of airplane engines. Patch declines to comment on Taggert, describing him as ill, but when pressed, admits the captain was perennially drunk. During a break, Patch tells John he must make sure that Taggert's letter read into evidence, as he suspects the engines were transferred in Rangoon to the China-bound ship, also owned by Gunderson. Back in court, Patch's credibility is questioned when his earlier suspension is revealed and he is accused of being responsible for the infirm Taggert's death. Patch asserts his suspicion of the cargo removal, but is dismayed when the owner's lawyer reveals a French salvage ship has located the Mary Deare . Patch acknowledges that he hid the location of the Mary Deare out of fear that the owners would try to intentionally sink the ship before the evidence of the missing cargo could be revealed. The court chairman dismisses Patch's outburst, and announces the inquiry will be adjourned pending the French salvage company's findings. Outside, Patch pleads with John to take him back the Mary Deare so that he can prove his suspicions, but John refuses. Janet then relates to Patch that she has overheard Gunderson ordering Higgins to join the salvage ship. That afternoon, John and Mike discover the Sea Witch missing, but later locate Patch refueling the vessel. Patch insists he must return to the Mary Deare not only to prove his theory, but because he fears a murder charge. Patch then explains that after the fire and explosion, he demanded Taggert order an investigation, but the captain refused. When Patch attempted his own investigation, Taggert threatened him at gunpoint, after which Patch struck him and Taggert died from falling into the cargo hold. Concerned that he may be charged with obstruction by the court, John agrees to return with Patch to the Minkies. When the Sea Witch nears the Mary Deare that evening, the men observe that Higgins has already arrived. John and Patch dive and swim unobserved through the battered bulkhead to the cargo hold where they find the engine boxes filled with heavy stones. Higgins spots Patch's underwater flashlight and attempts to trap the men in the hold and then harpoon them, but Patch tricks and attacks him. Meanwhile, concerned with John and Patch's lengthening absence, Mike comes on board the Mary Deare and, summoning the harbor police, has Higgins is arrested. Later, Patch thanks John for helping him clear his name.