HMS Terrier, a Royal Navywarship, is docked at Chatham Royal Dockyard. The officers are drinking gin and tonics in the captain's quarters while the crew are down below being given casual but technical lectures by Seaman Blissworth, who is very knowledgeable about torpedoes. An American Naval Commander arrives on board to discuss plans for a new "acoustic torpedo". Blissworth accidentally spills a drink on the secret plans and hangs it out to dry in front of a heater. He swaps the folded secret plan for other plans. The heater burns a hole in the secret plan. They try and hide the fact from a fiery visiting Admiral. The visiting Commander, now en-route to London has the only surviving plan. Blissworth is sent on a bicycle to catch him. This fails as he is not allowed out without a pass. Meanwhile Admiral Pettigrew wants to see the plans. He has already encountered Blissworth on his bike so stage one is to throw the bike overboard. It is decided to disguise Blissworth as a scientific expert, Professor Potter. In order to achieve this he has the petty officer's generous beard glued onto his chin, glasses are added and he adopts a Scottish accent. He convinces the visitor that the circuit diagram of a refrigeration unit is actually that of the torpedo. At the same time, the real scientist turns out to be female, Agatha Potter, and phones to say she will be late. The confusion causes her to be arrested as a suspected spy. Meanwhile Petty Officer Mundy is ordered to shave off the remains of his whiskers. Blissworth then is re-disguised as a woman: Agatha Potter. He is collected by the Admiral's secretary and taken to Admiralty House to meet the Admiral. He is there when the real Agatha calls and exposes him. He gets out and gets a lift with the returning American commander in his car, where he manages to get the correct torpedo plan back. Back on the ship he returns the plan and quickly gets changed back to uniform begore the admiral returns with the real professor. As he tries to sneak out he cannot help but interupt the discission and discovers the flaw in the current design. He is invited to work with the professor on the design. When the first torpedo is eventually fired in turns around and blows up their own ship.
