Walter, an inventor, is ejected from his home after an unsuccessful experiment has caused devastation. He takes a job in the mechanical toy department of a big store and after a series of misadventures in which he substitutes a real baby for a baby doll, he meets Pauline, a secretary at the War Office in whom he confides details of his latest invention - a wireless controlled tank. A child demands some toy balloons. In his efforts to wrap up the inflated balloons Walter sends mechanical toys running off through the store and is dismissed.

Walter shares lodgings with Cuthbert, an amiable waiter who resents Walter's high-handed treatment of him. Elated at his success in getting the model of his wireless-controlled tank to work, Walter dashes out, meets Pauline again and invites her in to watch a demonstration. He incurs Cuthberts's secret anger when he introduces him as his manservant and makes him serve a meal. Pauline offers to secure Walter an appointment with the War Minister.

The head of a hostile secret service has got wind of the invention and when Walter sets off for Whitehall with his model he is followed by spies to the tube station. Their efforts to obtain the model involve a chase up and down the spiral staircase, in and out of lifts and trains, but in the end Walter emerges triumphant. The chase continues but Walter takes refuge in the Imperial War Museum where he finds himself confronted by a wax tableau of a Cabinet meeting; thinking he has arrived at his destination, he starts to explain his invention.

His confidence gone as a result of the impassive silence of his audience, Walter dejectedly takes his leave and makes his way to the restaurant where Cuthbert works. He talks wildly of suicide and when his friend sees him taking some headache pills he immediately suspects the worst; police, doctors and ambulances are summoned and in the ensuing chaos Walter makes his escape and bumps once more into Pauline who rushes him off immediately to the War Office.

The day arrives when Walter must demonstrate his invention with a full-sized tank. From London the War Minister and Pauline set out to watch, but en route their car is held up by the spies and their chief disguises himself as the Minister and arrives at the demonstration in the official car. During the reception by the local Mayor, Cuthbert is snubbed by Walter and vows revenge.

Walter's first demonstration proves that he has the tank under control. When he is asked to repeat the performance, Cuthbert tries to reconnect the wires, but is knocked out by the spies. Regaining consciousness, he overhears them telephoning headquarters, and having enveloped them in the folds of a marquee, hastily reconnects the wires.

The tank is bearing down upon the barn in which Pauline and the War Minister are imprisoned when Walter makes one more desperate effort to stop it. To his surprise it stops dead. His invention a success, Walter and Pauline take their places on the tank which takes them to a church and crashes its way through the west door.

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