Oliver Reed and Michael Crawford play two brothers who are always trying to find some way to succeed with cleverness rather than simple drudgery. Crawford is constantly living in his brother's shadow as the one who gets caught. After Crawford is forced to resign from the army after an episode of unappreciated cleverness, the two decide their careers would go better if there was a large amount of publicity, so they decide to steal the crown jewels from the tower of London. Bombs, misdirection, disguises and acting allow them to enter the tower with all of the alarms turned off.

Architect David Tremayne and his brother Michael concoct a daring plan to "borrow" the British crown jewels from the Tower of London after Michael is expelled from military school. They prepare dated and sealed letters guaranteeing return of the jewels one week after the theft, then set their plan into motion by initiating a series of bomb scares in order to study the routine of Scotland Yard and the Army Bomb Disposal Unit. Next, they conceal a bomb in the Tower's Jewel Room, inform the police with an anonymous phone call, change into army uniforms, and join the bomb squad. Once inside, they chloroform the opposition, steal the jewels, cover themselves with fake blood, escape to a waiting ambulance, and quickly overpower the drivers. Authorities are completely baffled as the robbery becomes the talk of the nation, but David's guilt is exposed when his letter is opened the following week. Michael, however, has neglected to mail his letter, denies all knowledge of the crime, and has removed the jewels from their hiding place in David's flat. Only David is charged and remanded into custody, but he tricks Michael into believing that Scotland Yard has found a lie in his alibi; before surrendering, however, Michael places the jewels on the Statue of Justice scales 200 feet above Old Bailey. Reunited in a prison cell, the brothers begin plans for a daring escape.