1939. In a Merseyside shipyard, HMS Hercules nears completion, but there are fears of sabotage.

McDermott, boss of the Britannic shipbuilding company, is certain that there is no chance of saboteurs infiltrating the shipyard. However, policemen are to be secretly brought in before the launch. Fuller, an employee in McDermott's offices who is in league with the saboteurs, makes note of these latest developments.

George Carter, a War Reserve Policeman, asks Jane Gray, a young woman who works in a shop, to come to the police concert he is performing in that evening. She accepts his offer.

Brewster, a local theatre and amusement park owner, is secretly a saboteur. Underneath the Wall of Death, an attraction at Brewster's park, his men dig a tunnel leading underneath the shipyard, intending to plant dynamite beneath the Hercules and detonate it at the moment of its launch. Fuller passes on his information to Brewster, suggesting that one of the saboteurs infiltrate the police. Brewster's henchman, Shaw, is entered into a motorcycle test so that he can join the Flying Squad. George, also entered for the test, competes against Shaw for a position in the squad. George fails the test, and Shaw is admitted to the squad.

The date of the launch is changed. Realising that his tunnel will not be finished in time, Brewster decides that the launch must be delayed. He plots to blow up the ship's propeller blades, stored at Truscott's Warehouse, during the police concert.

George becomes an unwitting pawn in Brewster's plot when Shaw tells him that he must leave the concert to meet some plainclothes men at the Warehouse, and distract a night watchman so that they can gain entry. Unaware that the plainclothes men are actually saboteurs, George follows these instructions. Inside the warehouse with the villains, George finally realises he has been deceived.

Another policeman arrives at the warehouse and the force is alerted. They rush to the warehouse, and foil the plot. However, they think that George is responsible. George evades capture, meeting Jane at the shop the next day. George remembers that one of the saboteurs said that he worked at the theatre.

Discovering that the blades are to be moved to the shipyard, Brewster concocts a scheme to swap an empty lorry for the fully loaded one en route to its destination. This is successfully accomplished.

Jane and George seek the villain at the theatre. George is spotted by Brewster's men and seeks refuge. Hiding behind some scenery, he hears of the lorry swap plan. The stolen lorry full of propeller blades arrives at the theatre. George commandeers the vehicle, Jane climbs aboard and they drive away. However, Fuller meets them in the street and tricks George into handing over the lorry. Meeting the police, Fuller is forced to return the blades; however, he uses the opportunity to implicate George in the theft.

George returns to the police station. When he realises that he is under suspicion, he escapes with Jane in a police car. On the day of the launch, George seeks help from Brewster. Too late he realises that Brewster is a saboteur. Shaw takes George prisoner. He drives him away to be shot. Shaw tells George that the dynamite under the HMS Hercules will explode at exactly 3 o'clock, when the detonator underneath the Wall of Death is activated.

George escapes Shaw by leaping on to a passing lorry and drives off in a miniature car he finds inside. He heads for the Wall of Death and drives underneath. He smashes the bomb detonator; the criminals rush to repair it before 3 o'clock. Driving outside, he finds himself inside the cylindrical amusement, where Shaw chases him on a motorcycle. Finally driving away over the rooftops, George eventually ends up on some scaffolding above the shipyard, where he slides down a cable, interrupting the speeches on the launching platform. He launches the ship early; the dynamite explodes too late to destroy the ship. George catches Brewster in a large muddy puddle.