In England, a small-time diamond merchant (Charles Grodin) is unexpectedly offered the chance to supervise the purchase and cutting of an extremely large diamond to be named after its wealthy owner (Trevor Howard). When the diamond is stolen from him, he is blackmailed into pulling off a major heist at “The System,” located at 11 Harrowhouse Street, City of London with the help of his beautiful and wealthy girlfriend (Candice Bergen). The key figure in the theft, however, is the inside man, Watts (James Mason) who works in the vault at The System. Watts is dying of cancer and wants to leave his family financially secure.
Although “The System” has an elaborate network of defences and alarms against intruders, the robbery is carried out at night by gaining access to the roof from an adjacent property and threading a hosepipe down a conduit into the vault, where Watts uses it to vacuum up thousands of rough diamonds out of their drawers. The thieves leave before the robbery is discovered, and when found in the vault in the morning, Watts claims to have eaten the gems. Before he can confess, Watts deliberately swallows poison and dies at 11 Harrowhouse Street. Most of the loot is buried in concrete, to prevent it flooding the market.