Two booted-out brothers, David Tremayne [Oliver Reed] and Michael Tremayne [Michael Crawford], crave something “big” to prove themselves beyond the usual mischief. They enlist Inge [Lotte Tarp], David’s new love, and take her on a London tour that includes a stop at the Tower of London. Over dinner, they learn a curious legal loophole: theft cannot be charged unless there’s an intent to permanently deprive the owner. With that loophole in mind, David proposes a plan to steal the Crown Jewels, delivering letters in advance to show they do not intend permanent deprivation.
Michael feels a twinge of jealousy as David is seen as the “good” son while he is cast as the “bad” one, yet the letters go out and the plan moves forward. They stage a sequence of audacious tests: they plant a bomb at the Albert Memorial and watch the police procedure unfold, plant another at the lion cage in the London Zoo, and even blow up a ladies’ lavatory as a demonstration of nerve. David acquires a laser, and they add a bomb at the stock exchange, while David heads to an army base and records the procedures with a tape recorder.
On the day of the operation, Michael goes to the Tower’s jewel room and hides a bomb there, while David and Michael head to the army base, where they substitute themselves for the bomb disposal expert and his assistant. They ride with the army toward the Tower, then enter the jewel room and, using the laser, cut into the cabinets to seize the Crown Jewels. They trigger a small bomb and a smoke device, stagger out as if injured, and are taken away in an ambulance to hospital—along with the jewels.
A worldwide manhunt follows as the media frenzy explodes. About a week after the robbery, the letters are opened and handed to the police. The Crown Jewels have vanished from their hiding place, and David is arrested while Michael—who swears he knows nothing about the robbery—claims an airtight alibi. The authorities cannot break Michael’s story, so he is released while David is indicted and bail is refused. The police devise a plan to test Michael’s alibi, but he remains elusive. In a dramatic turn, Michael is shown digging up the jewels at Stonehenge, then makes a taunting call from a tapped line to say he will return the jewels at Trafalgar Square at 4 a.m. The police cordon the area, yet Michael uses their attention to place the jewels in the scales of justice atop the Old Bailey. The story closes with both brothers imprisoned in the Tower, quietly plotting their escape.
