During a routine response to a burglar alarm at premises in Long Acre, Central London, the police meet the nightwatchman and search the site, finding nothing amiss. The next day, a safe that had been opened with a key is found emptied. Superintendent Tom Halliday, Jack Hawkins and his new Detective Sergeant Ward, John Stratton, take up the case, convinced the culprit is a meticulous nightwatchman behind a string of break-ins.
Halliday soon deduces that the so-called nightwatchman has pulled off 14 safe-breaking jobs across the country, all on the same model of safe and all completed with genuine keys. A visit to the safe manufacturer yields a list of current and former staff, but everyone is cleared, narrowing the field of suspects. When another safe is breached, a young man on his way to work witnesses the thief climbing over the gate and, in a bid to stop the getaway, is knocked down by the car. The victim dies but manages to pass limited information to the police before his time, and the hit-and-run vehicle is found abandoned in a scrapyard, having been stolen from Mrs Elliot Ursula Howells.
Inside the car they discover a newspaper that points to a North Wales garage and to Mr Gilson, a deceased former employee of the safe manufacturer. Halliday uncovers that there are 28 more safes of the same type hidden around London, and that the thief is being tipped off by an insurance agent about which safes hold significant cash. To trap the culprit, the police arrange with the owner of a Royal Festival Hall safe to stage gala nights that would attract a large sum of cash stored overnight. They tail the insurance agent to a meeting with Mrs Elliot, who is revealed to be Mrs Gilson, the wife of the apparently dead key maker.
As the investigation tightens, Halliday and Ward realize that Gilson faked his own death after years spent making duplicate keys for the safes his company produced. Gilson finally makes a bold move, breaking into the Royal Festival Hall, only to be met by waiting detectives. Mrs Gilson arrives and waits in her car in a nearby car park. After a brief struggle, Gilson punches Ward and tries to flee, but is soon recaptured. In a daring turn, Halliday jumps onto the bonnet of Mrs Gilson’s car and smashes the windscreen to prevent her from steering to an escape route. He is thrown from the bonnet, but the arrest of both Mr and Mrs Gilson brings the case to a close.
