British and American sailors conduct demolitions experiments off the Scottish coast.

At a secret military operation off the coast of Scotland, on an island nicknamed "Sorrento," the Royal Navy is testing a new torpedo. When lead scientist Vincent and a young sailor are killed in an underwater test, the Admiralty decides to call in Lieutenant Bradville, an American Naval officer who is an expert in DPT, the explosive used in the torpedo. Some of the officers resent having Bradville join them, especially Lieutenant Wharton, who was Vincent's assistant. The four remaining seamen, Lofty Turner, Charlie Badger, Sprog Sims and Haggis Mackintosh, who have all volunteered, are also apprehensive, especially Badger, whose sweetheart left him for an American during the war. When Bradville arrives, Wharton considers his insistence on redoing everything that has been done in the past as an insult to Vincent's memory. The softspoken and humble Bradville tries not to antagonize Wharton, but remains firm. Bradville's team, sailors Butch Clelland and Shorty Karminsky, join their British counterparts. Although the group is uncomfortable, they start to adjust, except for Badger, who is openly hostile. Soon Bradville incurs Wharton's increased resentment when he has Butch and Shorty, rather than the British crew, disassemble the torpedo. That evening, Badger warms to the Americans when Bradville offers him a drink, along with Butch and Shorty, but when Badger hears that Butch's last name is Clelland, the same as the American who ran off with his sweetheart, he quickly leaves. Later, Badger confronts Butch and learns that he is, indeed, the man who married his girl, although Butch did not know about Badger. Badger takes a swing at Butch and a fight ensues, but is broken up by the appearance of Petty Officer Herbert. Herbert, a bully who has a grudge against Lofty from a previous assignment, gives the men a warning when they refuse to reveal who started the fight. The next day, a dry run test on the torpedo seems to indicate that everything is fine, but Bradville is still worried they have missed something. That night, in the sailors' quarters, Butch reveals that his wife is a drinker and manchaser, thus angering Badger even more. Sometime later, as the real test of the torpedo is about to take place in a small, two-man submarine, Wharton is furious when he learns that Bradville has been taking lessons in navigating the vessel. Thinking that Bradville's "grandstanding" will lessen Vincent's memory, Wharton angrily confronts him. Bradville tries to explain that it is his duty to assume the risks himself, just as Vincent had done, but the decision is taken out of their hands when Lt. Sterling is selected to sail the sub. The second man has to come from the British sailors, who decide to draw lots. Haggis, the most sensible and well-liked of the sailors, is selected, and as he prepares to leave, tries to convince Badger to get over his bitterness. During the test, which takes place a few miles offshore, Sterling presses the button to release the torpedo and the result of Vincent's test is repeated: just after leaving its chamber, the torpedo explodes, sinking the sub and killing Haggis and Sterling. The tragedy of the accident and a poignant letter Haggis left for the others, brings them closer together, and Wharton apologizes to Bradville for putting too much faith in Vincent's data. Bradville is certain that DPT is stable and did not cause the explosion, but is sure that he, too, must have missed something. The next day, a message is received from the Admiralty saying that all work on the new torpedo must cease, and the sailors, who have now become close, sadly pack their things. Sprog, the youngest of the group, admits to Lofty that he was afraid of being the one sent to man the sub. That night, Wharton wakes up with an inspiration and does some additional testing on another torpedo. When he looks at the torpedo's rudder, he gets an idea, and a few moments later an explosion is heard. Bradville and the other race to the building and find Wharton on the floor, but all right. Wharton reveals that the submarine test did not work, whereas the dry test did, because they had not accounted for the movements of the torpedo's guiding controls. Bradville soon goes to London to report the findings to the Admiralty, and they agree to allow one more test. Lofty, Sprog and Badger now draw lots to accompany Bradville on the test and Lofty secretly arranges to be chosen. Sprog guesses what has happened, but Lofty stops him from talking. When Herbert comes to get Lofty for the test, his verbal abuse is too much for Lofty, who knocks Herbert out. When Herbert comes to, the American and British sailors all say that Herbert struck Lofty, and Herbert angrily leaves, knowing that the officers would never believe him. Some hours later, after the sailors hear an explosion out at sea, they anxiously wait for news. Sprog guiltily reveals that Lofty rigged the selection, moments before Lofty comes back, happily informing them that the test was a success. A short time later, the British and American crews part as friends as Bradville, Butch and Shorty sail away.