The Long Arm

Police attend the premises in Long Acre Central London after a burglar alarm sounds. They meet the nightwatchman and are allowed to search the location. They find nothing amiss, but the incident sets up the later discovery that will trigger the investigation into the safe break-ins.

On the next day the safe opened with a genuine key is found empty. The apparent inside job suggests a fraudulent nightwatchman may be involved. Investigators begin to connect the incident to a wider pattern of safe thefts.

Superintendent Tom Halliday and Detective Sergeant Ward take on the case and start scouring for the fraudulent nightwatchman. They assemble timelines and examine patterns across different sites. The investigation expands beyond the initial premises.

Halliday concludes the false nightwatchman has carried out 14 safe breaking jobs across the country. All incidents involve the same style of safe and rely on genuine keys. The findings point to an insider with access at multiple locations.

A visit to the safe manufacturer yields a list of current and former staff but none are implicated. The cleared staff push the detectives to look for other leads. The mystery deepens as the motive remains unclear.

Another safe is broken into and a young man on the way to work sees the thief climbing over a gate. He tries to stop the thieves but is run over by the getaway car. The victim dies but manages to pass limited information to the police.

The getaway car is traced to Mrs Elliot, wife of a deceased safe key maker. In a scrapyard the car is found and a newspaper inside points investigators to a North Wales garage and to Mr Gilson.

Halliday finds there are 28 more safes of the same type in London. The scope of the operation expands, revealing a network of targets.

Police learn that the thief is being tipped off by an insurance agent about which safes hold cash. The plan is to coordinate with a safe owner at the Royal Festival Hall to stage gala nights and trap the thief.

The team tails the insurance agent to a meeting with Mrs Elliot, later identified as Mrs Gilson. She is the wife of the apparently dead key maker, linking the crime to the family.

Halliday and Ward deduce that Gilson faked his death and spent years making duplicate keys for all safes his company produced. The revelation ties together the earlier leads.

Gilson breaks into the Royal Festival Hall, but detectives are waiting. Mrs Gilson arrives and waits in her car in the nearby car park. After a short scuffle, Gilson punches Ward and tries to flee but is recaptured.