HMS Terrier, a Royal Navy warship, sits docked at Chatham Royal Dockyard, where the officers are gathered in the captain’s quarters, clutching gin and tonics, while the crew below receive casual but technically precise lectures from Seaman Blissworth Kenneth Connor, a sharp and knowledgeable torpedo expert. The mood of routine is about to collide with a bold new plan when an American Naval Commander arrives on board to discuss a project for a new acoustic torpedo.
Blissworth’s expertise is put to the test in a moment of comic misfortune: he accidentally spills a drink on the secret plans and hangs them out to dry in front of a heater. In a patient, chaotic scramble to salvage the situation, he swaps the folded secret plan for other plans (the ones for the ice unit), and the heater burns a hole in the secret diagram. The crew scrambles to hide the damage from a fiery and exacting Admiral, Adm. Sir Humphrey Pettigrew [Noel Purcell], who is bound to inspect the results of these “technical” efforts.
The visiting Commander is now on the way to London with what remains of the critical plan, the only surviving blueprint. Blissworth is dispatched on a bicycle to intercept him, but the bureaucratic rule that he must carry a pass prevents a quick escape. Meanwhile, Pettigrew grows more insistent about seeing the plans, and his pursuit of the elusive diagram leads to a dramatic misdirection: he’s already encountered Blissworth on the bike, and the plan is to throw the bike overboard to dispose of the problem.
To salvage the situation, Blissworth is transformed into a convincing scientific figure, adopting the disguise of Professor Potter. The transformation involves a substantial beard graft, new glasses, and a Scottish accent, all to persuade the Admiral’s party that he is a genuine expert. The ruse is complicated by the fact that the real scientist, Agatha Potter, is actually a woman and phones in to say she will be late. The confusion spirals: the illusion of Professor Potter is accepted long enough for Agatha to be misidentified and arrested as a suspected spy.
Complicating matters further, Petty Officer Mundy [Sid James] is ordered to remove the remaining whiskers, since the Navy only allows clean-shaven or fully whiskered looks, adding another layer of comic tension to the deception.
Blissworth then re-emerges in a bold new disguise, appearing as a woman again—Agatha Potter. He is collected by the Admiral’s secretary [Peter Howell] and brought to Admiralty House, where the façade stands under scrutiny. He is present when the real Agatha calls and exposes the ruse, but the moment of revelation passes with a narrow escape.
Escaping the scrutiny, Blissworth secures a ride with the returning American commander and manages to recover the correct torpedo plan. Back aboard HMS Terrier, he returns the plan and quickly shifts back into his uniform before the Admiral returns with the real professor. Yet the tension of the misdirection lingers, and Blissworth cannot entirely slip away from the conversation without shedding light on the design’s flaw.
In a surprising turn, Blissworth is invited to work with the real professor on the torpedo’s design. The first test, however, proves fateful: the torpedo fires, but instead of delivering the strike, it turns around and blows up the very ship it is meant to glorify, leaving the crew to reflect on a mission that began with bravado and ended in an unintended catastrophe.
